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.
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.