Sources
The Observer's Pocket Series

This page is for those who want to start collecting and want to know how and where to start hunting, so get your wallet ready and read on.
Observer's Books published by Warne can only be obtained in used (second hand) condition as they are no longer published or available new.
To build your collection you will have to rely on the book trade, other collectors or the very hard work of hunting down discarded books at a jumble or boot sale.  Most book dealers are of the opinion that sources for discarded books have all but dried up as more people become aware of their collectable value.
Observer's Books were produced and sold in very large numbers and were very popular, it is unlikely that many would have been thrown away, so there should still be a plentiful supply of these little beauties around.

The following sources may prove fruitful.

Internet
Shops
Book Fairs
Boot Sales
Collectors

Internet
If you don't want to travel around, internet shopping has got to be the answer.
Look for items within your own country as shipping costs may be more than the cost of a single book purchased from overseas.

Online Book Stores.
Many used book stores have their inventory published on the web, the best method is to use one of the book search engines, this should turn up several hundred for sale world-wide in all but the rarer titles.  A brief description of condition may be available online but you will have to make enquiries to the individual bookseller for a more detailed description, these lists can also be considerably out of date and the better examples will be snapped up within a couple of days. 
This 1975 first edition Golf came from a bookshop in British Columbia, Canada, it turned out to be in absolute pristine condition, costing just $5 (around £3), some additional shipping charges brings the price up a bit though. 

One of the best book search engines...........
http://www.abebooks.com

Price range £3.00 ($5) upwards.

Online Auctions.
This can be real fun and some of the rarer titles do turn up, don't expect to get bargains though, competition from other collectors can be fierce especially for the more desirable items, and you may end up paying a great deal more than the book is worth or costing more than you can afford, but there is always a chance to win a bargain on the more common titles especially if you can put up with a book in something less than perfect condition. 
Ebay seems the most popular www.ebay.co.uk but other on-line auction sites may turn up something.   Read the description carefully and take a close look at the picture if provided, noting any apparent faults, if in doubt - ask the seller, who will be glad to provide further details.
This rare title 'Tennis' in excellent condition was purchased for more than £30 at auction, bidding can be tough if two (or more) people really want the same item. Just twenty years ago it would have cost a measly couple of pounds new.

Reserved prices usually start around £2-£5 or $5-$10

Despite the popularity and convenience of buying online, I have no doubt that handling and inspecting a used book is a far better prospect, this is especially so if you are collecting books of a certain quality, no amount of description or pictures can replace inspecting the book yourself.

Shops

Second Hand Book Shops.
Most large towns have one, some of the smaller ones too.  Not all shops will carry any stock of Observer's Books so a telephone call may save you time and a wasted journey.  Unfortunately these shops have probably been visited by collectors already and the best stuff has been filtered out, unless you are lucky, the remainder is likely to be the common titles and some may be of dubious quality or highly priced.
Book dealers are pretty well informed and are not likely to overlook rarities so don't always expect bargains - but it is possible.
This Sculpture was the only printing of this title in 1966 but has the rare jacket added around 1972, guess it was a lucky find, purchased for just £5.00 from a local bookshop and in excellent condition.

Price range £3.00 upwards

Charity Shops.
A Charity Shop here in the U.K. sells unwanted used goods which have been donated - clothes, bric-a-brac and usually some books, the proceeds go to their particular charity.  You would have to be lucky to happen upon anything here but I have done so on several occasions.
You never know what may turn up...it's likely to cost under a quid (£1) too, although in the last couple of years the Charity Shops are catching up with the prices you may have to pay at a second-hand bookshop!
Price range £0.50 - £3.00

This delightful 1966 edition of Ferns popped out of a Charity Shop bookshelf in Devon for £3.00, although a rather high price from this source...I did not hesitate as it's a worthwhile cause anyway.

Junk Shops.
Worth a try, some do carry used books but so far I have not turned up anything in good condition. 

Book Fairs

Book Fairs are a collection of traders/dealers assembled at one venue, this is predominately for collectors and bibliophiles, usually a large amount of used and collectable  books for all interests can be viewed.  Some of these dealers are likely to have a few Observer's Books for sale.
I have only experience of one or two of my local book fairs which are usually held every month, there is always a good selection of Observer's titles available but not always the rarer ones. 

I have obtained many quality books at very reasonable prices, most dealers are prepared to haggle on more expensive items.  Book Fair dealers often do not have the overheads of a bookshop and are probably the best source for variety, quality and quantity.  Inevitably some books will be overpriced, but just occasionally a real gem or a bargain will turn up.
Price range £2.50 upwards

Boot Sales

Car Boot Sales are my personal favourite.  Boot Sales have proliferated here in the U.K over the last ten years or so, it is a great Sunday morning pastime, usually held in a field away in the countryside. 
For foreign visitors I will explain that a "Car Boot Sale" is a number of sellers getting rid of their smaller unwanted household items which can be transported to the sale site in their vehicles, goods are usually at bargain prices, most items will be in used condition and sold without any guarantee of course.  If you search long enough and hard enough you should come across just about anything at a boot sale, this really is collectors paradise! but there is a lot of competition and it's essential to be an early riser.
There are always plenty of general used books both paperbacks and bound varieties available but it is hard work searching through these large amounts of jumble.  Fortunately Observer's are easily recognisable and by others too! so it's likely that they will be snapped up quickly.
 
I have picked up many bargains this way including this 1966 first edition Furniture for just 10p found at the bottom of a box of book jumble.

Probably my best find was a fine copy of 'Vintage Cars' for 30p!!
A lot of sellers have got the message and are trying to pass on really rough examples without DJ's for £3 or more - ignore.
Price range £0.10 - £3.00 

Collectors

Other collectors often have swaps or books for sale, usually duplicates from their collection, it's likely that they will have a list of such and will be glad to let you see it.

Lastly, ask friends and relatives to check their attics, it could be your biggest bargain ever.

GOOD HUNTING.