Pasteurized human donor milk is flipping expensive. We use it extremely judiciously in our NICU. There are protocols up the wazoo for its use, consent forms for parents to sign, and we have to treat it as though it is a blood product.
It's difficult to recruit donors, and part of the problem is that milk banks are not widely available in North America. I think there are maybe 3 or 4 that supply all of Canada's NICUs.
I don't know if a cross-over agency (milk bank + blood donation service) is a feasible option, given that its only purpose would be to supply leukocyte infusions and fractions to JWs.
Then there's the problem of extracting viable leukocytes from breast milk - would it require pasteurization or some other form of processing? How far do you want to go with obtaining fractions?
FWIW, the richest supply of leukocytes in human milk occurs mostly in the first few days of milk production, with colostrum. Most women would want their own babies to have that limited supply, not some random, thankless JW who can't even be arsed to donate their own breastmilk or blood to help other people.