I used to use one from Badge-A-Minit that I got for 99.95 with a bunch of parts. I'm hesitant to recommend them though, since in my experience (and in many of the reviews I've read), the machine has an unacceptably high failure rate. No matter how careful you are, you're going to get quite a few defective buttons. I recently passed that button maker on, and upgraded to an American Button Machine. It was expensive, but I can't say enough good things about it!
I know that there are a few other, cheaper machines, but I'm pretty sure they're aimed more toward kids as toys. Unfortunately, it seems like you really get what you pay for when it comes to button makers. You might want to look into renting one, if you can. I have a friend who did that for a convention, and he said it worked out well. I'm not exactly sure how one would go about that, but you might search around online, or something.
