What you're looking for isn't paint, you want RIT die. It works a treat.
That being said, it's a royal pain in the rear end, and makes the house smell like cabbage for a few days, so patrick's advice is a pretty good one.
That being said, you should NOT try to buy from alienware or falcon NW - even if they would sell a custom keyboard, they aren't going to be that great of quality.
If you want one done for you, and don't care about the price, check out WSAD keyboards. If you want to be really specific about it, or if price matters, go with RIT dye.
If you aren't doing this on a mechanical keyboard, then really... your priorities are in a very odd order.