These are good online stores for Mac compatible RAM
OWC http://eshop.macsales.com/shop/memory/Apple_MacBook_MacBook_Pro/Upgrade/DDR3_Whi te - They offer Mac tested RAM at very good prices.
Crucial Memory http://www.crucial.com/ - good place to buy RAM from all over the world. They also have an excellent memory selector that allows you to choose memory based on your computer's model
Data Memory Systems http://www.datamemorysystems.com/apple-memory.asp - another good, cheap place to buying RAM if you live in the U.S.
Here are video instructions on replacing the RAM on the White Unibody.
If you don’t have the tools to open up the MacBook OWC has a set for $5.