There's no insanely good farm other than hoping they spawn close to some of your Lighthouses. Unless you are willing to do some manual grinding that is.
The Winter Dungeon has a lot of berries in it, and this is a good strategy for farming berries in Forager. What you want to do is to just run in, solve the dungeon and loot as many berries as possible, then run out to reset it. If you have a fully upgraded Amulet, you should be getting around 150 - 200 Berries per run, and each run should only take a minute or so.
This means that you won't realistically be able to grind millions of Berries, but it is entirely doable to farm a few thousand of them in a somewhat short amount of time.
Can you plant Berries in Forager?Right you you won't be able to plant berries to set up a Berry farm, but this is something that the developer of Forager has said that he is working in implementing. So you will likely be able to craft Berry Seeds at the Mill once we get an update to the game.
When this is possible, it should be pretty easy to stack Lighthouses next to the Berry farm to get lots and lots of Berries in a short time.