You cant they lose potency very quickly, Currently they may only be grown in Africa.

They are actually found wild in the Amazon rainforest. But yes they can't be bought fresh because they spoil too quickly. There is a brand, called Amazon Mania, that sells packs of raw frozen acai pulp, they are really really good and as close to the real deal as you can get. sometimes whole foods has them, or health stores. you can google the company.