I tried to buy breastmilk online, and I googled a lot, without success. I couldn't find any serious website, I seriously don't know anyone who bough or sold brestmilk. When you search online it seems like lots of people buy and sell because there's so much discussion about it, but I seriously never saw any website actually offering the service. I think you would need to find yourself someone in your area who needs breastmilk, you can't really ship it because it's gonna get spoiled.
It's most likely illegal, only milk banks are allowed. But if you're doing a private transaction with someone, who'se gonna know?