In FO3 or New Vegas? In Fallout 3 it's in vending machines all over, for free. In New Vegas it's rarer, but you can find it in random fridges and vending machines in some of the offices at the edge of Vegas. There are free bottles in the Lucky 38 suite as well as the presidential suite at the Tops. If you're looking to buy, just find vendors who stock food and give you a good deal. If you fixed her radio and took care of Joe Cobb, I think Trudy in Goodsprings will sell it roughly at cost.Check Nuka Cola vendors for free Nuka Cola.