What did the Israelite's flocks eat in the wilderness?

Answer 1

Manna fell from the sky.

Jewish answer:

1) They weren't "wandering." They journeyed and encamped according to God's explicit guidance (Exodus ch.13, Numbers ch.9).

2) They didn't "find" food. God directly sent them the manna (Exodus ch.16). On two occasions, He also brought them birds (quail) to eat (Exodus ch.16, Numbers ch.11).

Answer 2

Then the LORD said to Moses, "Behold, I will rain bread from heaven for you

And the LORD spoke to Moses, saying, "I have heard the complaints of the children of Israel. Speak to them, saying, 'At twilight you shall eat meat, and in the morning you shall be filled with bread. And you shall know that I am the LORD your God.'"

So it was that quails came up at evening and covered the camp, and in the morning the dew lay all around the camp. And when the layer of dew lifted, there, on the surface of the wilderness, was a small round substance, as fine as frost on the ground. 15 So when the children of Israel saw it, they said to one another, "What is it?" For they did not know what it was. And Moses said to them, "This is the bread which the LORD has given you to eat. And the house of Israel called its name Manna. And it was like white coriander seed, and the taste of it was like wafers made with honey.

Answer 3

"Wilderness" is not synonymous with "desert." In the wilderness there was enough wildgrass for the flocks to survive.