So Anthropologie is the easiest answer when it comes to shopping for cute knobs. But I need 7 knobs for my dresser and at Anthro they cost 6 to 12 bucks. So that's like 50 bucks, at the very least, on new hardware. Yikes.
Here are a couple other resources I found, Cool Knobs and Pulls and House of Antique Hardware.
All of these are in the $2 - $4 range:
UPDATE: Commenters pointed out Hobby Lobby also has great knobs for cheap. You guys are geniuses.