well first and formost I say your local LHS.. if that fails A Main Hobbies has a good selection of K hop ups.
http://www.amainhobbies.com/index.php?cPath=24_64
-- John --
1:1.. 01 Z28 Camaro A4 Sure.. its stock.
1:8's.. K3, Inferno GT, and a straight line beater car
1:18.. RC18t Mamba and 3s lipo