1- Target. Target has a wide variety of sexy, classic, sweet and stylish bras at great prices. Try on a big variety to find the perfect cut and style for you.
2- Places like TJ Maxx, Marshalls and Ross all have great designer bras for super cheap prices. From DKNY to Wonder Bra and Calvin Klein, you can stumble across many different high quality brand bras. You just need to be willing to dig a little.
The trick is to always have your eye out and try a variety of stores to find a bra that suits your needs. Good luck!