speaking of checking book sites for TSR items, i've have some luck in the past checking out half.com. picked up quite a few early issues of dungeon that way (although i still need #4 and #5), and it's pretty easy to find common modules at a decent price, you just have to keep checking. people will always have them up for $8 or $10, but occasionly you can pick up a decent module for $4. i actually bought copies of
H2,
H3, and
H4 on half.com for $10-15 each.
i was also lucky enough to find a personally owned store online based in washington state one day, bought a copy of WGR1 for $15.