Buffy says it is hard enough as it is to find jeans that fit right. But she has completely given up hope of ever finding a bra.
One good side effect about the slayer healing powers though, her tits won't ever sag and will be firmer then teenage breasts.
So she won't need a bra, although she has stretched out every shirt or top she has. But is it cold in there or is Angel working out with his shirt off?