Welcome to lw! In the Goldsea den there's an event (Cooking: Find Ingredients) that will give you 2 random cooking ingredients, and milk is one of the possible items! I think the event only becomes available once your cooking level hits 20. I have plenty stocked up and can send you some to unlock the recipes, though! :)
I was just wondering where to get an indecent amount of milk hoping to find another way that it's not just the Goldsea event, Mole market occasionally or the cooking event shop ;D (I'm cooking a ton of food for wolf training/Murkwood event and making strawberry yogurt is the fastest option lol)