There are a lot of places to buy ASL-related items.  If you want to try to find stuff locally, the first place to check would be your local hobby and gaming stores.  Major toy retailers like Toys 'R Us do not carry a niche product like ASL, but many hobby and gaming stores (and even a few comic book stores) do. 

However, one of the easiest ways to find ASL stuff is on-line, starting with the Multi-Man Publishing Web site at multimanpublishing.com.  Here you can buy the latest stuff, pre-order new stuff, and even take advantage of occasional cool sales. 

In addition, there are several on-line retailers who typically carry a lot of ASL items.  One of the best is the Gamer's Armory, run by Scott Blanton, at www.gamersarmory.com.  He has good prices and a reputation for good customer service.

Sometimes you can find ASL stuff at Boulder Games at www.bouldergames.com.  Fine Games at www.finegames.com has new ASL stuff but also often out-of-print ASL items as well.

British gamers might try Infinity Games UK at www.iguk.co.uk, Leisure Games at www.leisuregames.com, or Spirit Games at www.spiritgames.co.uk.  Second Chance Games at www.secondchancegames.com also usually has ASL stuff, including used items.  Great Britain actually has a wealth of on-line retailers that carry at least some ASL items

Australians might try Mil Sims Games at www.milsims.com.au.  Canadians can check out Imperial Hobbies at www.imperialhobbies.ca.  French-speakers can try Histoire et Figurine at www.histofig.com.  Germans can look for ASL stuff at Fantasy Encounter at www.fanen.com.  Folks from Japan can try the Internet Online Game Market at www.boardwalk.co.jp.   These are all just possibilities; a few Google searches and it is possible to find a great many on-line retailers who stock ASL items.

For people searching for "third party" ASL items, sometimes one can find them at on-line retail stores, but only a few such stores, such as Gamers Armory (www.gamersarmory.com) tend to stock a lot of items.  Otherwise, it is best to go to the Web site of the publisher and order it directly there, or to use E-bay.

And speaking of E-bay, last but far from least, this on-line auction site always has a lot of new and old, used and unused ASL items for sale at any given time.  Bargains can be had, but arms and legs are also sometimes at risk, so caveat emptor.  Nevertheless, E-bay is hands down one of the quickest ways to build up an ASL collection, and often the only way to acquire rare out-of-print ASL-related items.