You might try seeing if Hot Topic still sells white foundation, I'm not sure if they do or not since I've not been in that store in ages. Otherwise you could check with Halloween/party stores to see if they have any. Lastly, online suppliers of stage makeup will likely have it, but depending on the brand it may be pricey. You can mix the white foundation with any other regular foundation to lighten it up.
Alternatively, if you don't feel like going through the trouble of mixing foundations and if you don't need too much lightening you can achieve a degree of lightening through using white powder (check the same places that I mentioned for the foundation) over whatever foundation you currently have. This is something i used to have to do before I started actually getting some sunlight.
If you go the powder route, make sure that the powder you're getting is actually white in color, not just in appearance. I mention this because a number of brands make 'mattifying' powders which often appear to be white in the package, but do not actually apply that way.