You absolutely can! You can look on a local Vegan moms page and ask for milk there, or search on a donor group. Many babies have sensitivities to certain ingredients, so it’s not at all uncommon to be able to find a donor who excludes ingredients from their diet (at least while breast-feeding).
But, especially for those of us who cannot get vegan formula, I would argue that even breastmilk from an omnivore or vegetarian (freely given) is as Vegan friendly (if not more Vegan) than buying a non-vegan formula. And, I would argue it is the more ideal nutrition for a baby human.
(please don’t take this as an attack or shaming towards anyone. We all do what we think is best. I just want to make sure people searching for options have all of them. Often times I see donor breastmilk being overlooked as an option)