You can get milk by milking the cows of Olvia and Falres dirt farm. It get 5 energy but you can obtain 6-10 milk each time.

Make 200 with 90 energy

If you don't want to gather milk by yourself, you can rent a fence (the same used to grow plants) and put haystack (they count as 4 plants in space), the haystack attracts cows and you gather them like they were plants, and give you milk, meat, etc.

Gathering Beginner: 25 - 40
Gathering Professional: 35 - 55
Gathering Master: 50 - 65