I doubt you will be able to get milk from a milk bank, as there was an article in the Denver Post about the national shortage of breast milk donation to milk banks. They don't even have for the needs of preemies, babies who have had surgery on there intestines, and babies that can't digest any known formulas. Even if you could find a bank with enough milk, you would be paying about 3-6 dollars an ounce. It would be $12-24 for one bottle!!
I used Milk Share, it is a site run by midwives to match people who have a surplus of milk and people who need milk. My two children never had formula despite my meeting 1/4 of their need at my peak. I had 6 main donors that I used along with probably 3 other one-time donors. I asked them for the labs from their pregnancy and then talked to them for a little while in person and online. I felt very comfortable that all my donors were kind hearted women who had my baby and their own child's best interest at heart. You need to be comfortable with them. I guess because I did surrogacy for both my children, I had to learn to trust other people to care for my children when I couldn't do it.
I would try to get on a milk exchange and try to get some milk stored away ahead of time. That was one of the best things I did with my son after having some panicking times when our supply of milk got low with my daughter.