Aside from new releases (within the last six months or so), there is no reliable source that deals strictly with complete MVS kits. Your best bet is to keep your eyes open on the selling board, sometime someone comes into a bunch of complete kits and they go there to sell them off. If you're looking for something specific post in the wanted section.
Other than that, patience will be your friend. Don't expect to type in a website or place a bid whenever you decide you want a certain complete kit, you will have to look for them.
I guess that's half the fun of dealing only with complete kits. Your collection will never be as big, but your buys will be spaced out and the hunt is half the fun (or so I hear. I'm too damned lazy and spoiled).