Norton Records (www.nortonrecords.com) has also put out a lot of great comps over the years. I'd highly recommend the Ft. Worth Teen Scene comps as a good starting point (not to mention their Sonics/Wailers reissues). They should also stock all of the Crypt Records comps mentioned above (Teenage Shutdown, Back From the Grave, Garage Punk Unknowns). Crypt doesn't have an online store for U.S. customers anymore, so your best bet in finding that stuff is through Norton, IMHO. The Pebbles comps can be found at www.bomp.com (they're on AIP, a subsidiary of Bomp Records). Sundazed also issued some killer '60s comps in their "Garage Beat '66" series. Here's Vol. 1: http://www.sundazed.com/shop/product_info.php?products_id=679
Mike Stax's Ugly Things magazine is an excellent resource, too. His site also has a Garage Compilation Database that can come in handy when trying to track down which comps have certain songs.
(Just click on "60's Come Database" in the menu on the right-hand side.)