Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
I recommend any manufacture/website that guarantees their RAM to work with your Mac. My personal recommendations are Crucial or DataMem. Crucial may be the best RAM on the market. DataMem is usually slightly cheeper and pretty damn good RAM as well.
 
I don't have an ebay account but if you put that up on Amazon Marketplace I'd buy it.
 
Did you just purchase a new Mac Mini with 512 MB RAM? I'm pretty sure it only has 1 RAM slot. The only way to upgrade it is to get a 1 GB stick and replace your 512.
 
I've used memory from 18004memory.com in 4 different Macs and they've all worked great. Great prices too.

I'm about to order a 1GB chip from them any day now for my iBook.
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.