local home brew supply store should be an obvious first stop if you have one. having exhausted that I would say between ebay, this forum, and google you shuld be all set!
so what are you building?
full bar with multiple taplines and a kegerator?
fridge conversion?
jockey box?
bucket 'o' ice with akeg in it?
easiest (cheapest) start up is a 5# co2 tank, single regulator, one corny keg, and a picknic tap. all this can be had easily < $200