I receive the quarterly catalog from The HitPointe. I also occasionally order books from them as a large order equates to some savings. I use the catalog as a reference for my library also. They pretty much have EVERYTHING.
Be warned, the site is outdated, but they have informed me they are working on a "Newer, more modern site." Best contact through email to customer service. They have been quick to reply and friendly folks. you can also use snail mail to the following address:
540 North Lapeer Rd.
Orion Township, MI 48362
There are many other complete resources available on the net and from back issues of the print editions of both Dungeon and Dragon Magazine. Good luck.
PS: Another resource I have been using as of late is RPG Drivethrough.
The site is down for maintenence as of this posting, but should be back up soon. You can purchase most anything fairly affordable in PDF format for instant download. ***WARNING*** The site has made me wish I had invested in a laser printer.