Look for a Hobbytown USA for the Tamiya. I've one just around the corner, and they stay pretty well stocked with Tamiya products.
As for the Vallejo Game Color, I really couldn't tell you other than your local game store, provided you have one where you live. The Hobbytown here stocks Vallejo's Model Air line but that's about it.
Edit: Oh, online stores. Well, I use thewarstore.com and frpgames.com quite a bit. Funny enough, it also looks like Amazon.com stocks Tamiya