Breed. Just take your evolved Pokemon with a Ditto to the Pokemon Day-Care (Route 3). Leave both there and just walk some steps. Then, go to that old guy outside the Day-Care and if you have an empty party slot, he will give you a Pokemon Egg. After you get the Egg, just walk around with it in your party and after a lot of walking, the Egg will hatch. And from the Egg will hatch... THE PRE-EVOLVED FORM OF THE POKEMON YOU LEFT IN THE DAY-CARE WITH DITTO!!! Works for almost all the Pokemon, except Legendaries.
*Please note that you have to beat the Elite 4 in order to get access to Route 3 and the Day-Care in B2/W2 and BB2/VW2.