Wet nurses exist. Period. I know of more than one woman who donates breast milk. They both have continued to pump as their kids grow to give back to the community.
Wet nurses are extremely expensive, they also introduce a variety of challenges in modern times as you must trust a stranger to carefully and correctly preserve the milk, not dilute it with anything, and be in good health without any complicating factors.
When all is said and done, milk can be purchased peer to peer at roughly 4x the cost of infant formula or between 1 and 2 dollars per oz. Purchasing through a service with some notional guarantees on the above concerns increases the cost to 5 dollars per oz. A 3-6 month old can consume 40+ oz per day.
“Wet nurse” is a phrase for this very reason.