I normally buy from Tower Hobbies and on Ebay! I know that Tower is not always the cheapest but they have great service in my Book! Ebay can be much cheaper then anywhere else but, that all depends on the bidding! If you are going to get a new air filter just go and get a Motor Saver!
http://www2.towerhobbies.com/cgi-bin...?&I=LXDNV3&P=7