I believe you would have to either level up a Mart in your Join Avenue (after 3rd badge) or grind PWT (after 5th IIRC maybe before) for BP and buy them @ 1 BP each.
I assume you meant places you could acquire mass amounts, I don't have a list of all the ones you find on the ground anyway lol.
If you go the Mart/Join Avenue route there are 2 different types of Marts that sell 3 different kinds of Vitamins each. (I recommend Serebii's article on it for specifics.) The vitamins are unlocked at rank 5, 6 and 7 Marts which isn't too bad. If you level them in conjunction with Dojo's/Cafes you save tons of time in the long run anyway as they both train EVs in mass amounts cheaply!