Boosters tend to cost more than the expected value of the pieces inside. Even if you get lucky every once in a while, they aren't cost effective.
Ebay is a pretty good source, as well as the buy/sell Star Wars Minis group on facebook. There are also good sellers on here. Check out bo_mastar's thread
http://www.bloomilk.com/Forums/default.aspx?g=posts&m=245039#245039
And jak is an active seller here, too.