Hi L.,
I donated breastmilk through the International Breast Milk Project founded by Jill Youse last year and thought they were great. You go through a screening process which includes a long questionaire and a blood screening (paid for by them and not a hassle). They also verify you have a quality freezer to store the milk. When you become a qualified donor, you can just do a one-time donation and send in what you already have stored or continue to send in milk as you collect it. They are flexible. You just request a shipping container when you have enough to ship and they pay all shipping costs and pick it up at your front door!
I believe they work with a company called Prolacta Bioscience and also are part of a project that sends donated breastmilk to South Africa.
This isn't the only option, but I had a great experience and would donate through them again.