Mac mini memory module
You can find the specifications for your computer here.

Specifically, the unit has the following (with a maximum RAM capacity of 8GB):


RAM Type: PC3-8500 DDR3 SO-DIMMs
Min. RAM Speed: 1066 MHz


So buying RAM that meets those specs is sufficient. As for buying "Apple" branded memory, it doesn't seem necessary. I always look to Mushkin, which sells "Apple" tested memory and have relied on them exclusively, but I have read others running Macs with Kingston, Crucial, or any one of the many other brands available without reporting issues.

Match the type and speed, and you shouldn't have any problem. Newegg has a 4GB package (2x2GB sticks) and a 8GB package (2x4GB sticks) for relatively cheap. They should give you an idea of what kind of prices to expect. Searching google yields quite a few results if you are interested in other brands or other configurations.

Lastly, consult iFixit for details on how to upgrade the memory for your specific model.