The earliest editions of the YW books are of interest to many collectors because of their relative rarity. The first hardcover runs, in particular, were fairly small, and most of them were sold to libraries, so that it’s hard to find any of the first three books in mint or even “very good” condition. So if you’re hunting for a match to that copy of So You Want to Be a Wizard that snagged your finger in the library, and you’re insistent on the best condition, you may have your work cut out for you.
Some of the larger sources which are worth checking if you’re hunting for one of these books are:
Alibris BookFetch | AbeBooks | TomFolio.com
Also worth checking are some of the online specialist children’s bookstores. They include:
Aleph-Bet Books | Children’s Pages | Jemma’s Books
Justin G. Schiller, Ltd. | BookRescue.com | Orrin Schwab Books
D&M Books | Windmill Books
UK users are in a slightly different situation from US readers, as the hardcovers are only very rarely to be found there, but the Corgi paperbacks are not so difficult to track down. UK book-hunters should check:
The Lion and Unicorn Bookshop | MunglesJungle.co.uk
Tales on Moon Lane | KidzBooks.co.uk
And of course no one should ignore the famous used book stores of Hay-on-Wye, the world’s first “Used Book Town.” Special attention to children’s books is given there by The Children’s Bookshop and Richard Booth Books.
Finally, check this long list of local UK children’s bookstore resources at Booktrusted.co.uk, and this extensive resource list at UKChildrensBooks.co.uk.
What’s the very rarest of the YW books? Hard to say. If it’s perfect condition you’re looking for, then probably the first edition of So You Want to Be a Wizard would win the prize. This edition has become extremely collectible not only due to its rarity (only 1500 copies were printed) but because of the cover illustration by David Wiesner, whose early cover works are now much prized. Copies in “very good” or better condition are routinely offered for upwards of USD $250. They tend not to come on the market very often, and the determined collector must either keep checking the usual sources or employ a “search service” to find one.
Otherwise, the rarest edition — in terms of “the hardest to find” — would probably be one or the other of the Japanese editions of 1991-1992. These illustrated editions of So You Want to Be a Wizard and Deep Wizardry were published in paperback by Dragon Books, an 
imprint of Fujumi; they are linked to here and here at Amazon.co.jp. (You can use AltaVista’s Babelfish to translate the pages.) The translations of the books were done by Ooide Ken.
Close behind these in the “hard-to-find” stakes would be the first Russian editions — at least, the first genuine editions: there are apparently many “samizdat” versions of the Armada editions floating around the territory which was once the USSR. These versions of So You Want To Be a Wizard (ISBN 5-87994-076-6), Deep Wizardry (ISBN 5-7632-0048-9), High Wizardry ((ISBN 5-87994-077-2) and A Wizard Abroad (ISBN 5-7632-0177-9) are illustrated in an exuberant and rather goofy style that almost makes them worth having for the pictures alone.
