It's not a stupid question. Usually they would have resorted to cow or goat milk, or some other animal. There is a long history of allowing babies to suckle directly from animals if a human female isn't available to provide milk.

In her book "Milk: A Local and Global History", Deborah Valenze writes that in the 18th century it was typical for French mothers to allow their babies to suckle from goats, and that orphans were sometimes held to the udders of animals kept on site just for feeding.

Valenze asserts that "Animal nursing rescued European societies more than once from widespread infant death." She cites the example of a doctor who visited an infant hospital in Aix-en-Provence in the early 1800s as follows:

“The cribs are arranged in a large room in 2 ranks. Each goat which comes to feed enters bleating and goes to hunt the infant which has been given it, pushes back the covering with its horns and straddles the crib to give such to the infant.”

Pierre Brouzet, the physician to King Louis the XV, wrote that he had known “some peasants who have no other nurses but ewes, and these peasants were as strong and vigorous as others."

In Nicholas Day’s book “Baby Meets World“ he cites a mid-19th century French volume on midwifery that declares goat nipples to be well-shaped for infants, and that goats bonded well with the children nursing from them.

A German book from 1816 was titled "The Goat as the Best and Most Agreeable Wet-Nurse."

Some Europeans preferred donkeys over goats for such purposes. Samuel X. Radbill, an American physician who researched the history of animal-human breastfeeding in 1976, writes: “Since they had a better moral reputation than goats, the children were less apt to acquire a libidinous character." He goes on to say that donkey milk was believed to cure distemper and poisoning and that pigs also often served as wet nurses, but they were “socially persona non grata.”