While there are a very small chance of getting it by fishing in various lakes the chance are there and it has nothing to do with exactly where on the shore you are standing. Handiness is easy to build up using the woodworking table and performing upgrades on all the things you can upgrade. If you have Cottage living it will also occasionally be for sale in one of the stalls in Henford.
You can also grow the cow berry by splicing Dragonfruit and Snapdragon plants.
Hope this helps.
Yeah, I get a lot of cowplant berries fishing in the farthest back fishing spot in Oasis Springs, but that's luck based (RNG). This map is from simsonline.info:
The most reliable way is to, as @CGrant56 said, graft dragonfruit and snapdragon. I'll also add that you're more likely to get cowberries if you graft the snapdragon onto the dragonfruit, instead of vice versa. This is because the rate of spawn depends on the rarity, so snapdragons with dragonfruit spliced on produce a lot of snapdragons because they're more common than the other two.
If the next question is "okay, so where do I get dragonfruit?" you need to graft strawberry and snapdragon. OR you can go to the Magic Realm's potion ingredient shop (if you have RoM) or the garden stalls on Henford-on-Bagley. Warning: they're expensive.
If you join the Gardener career in the Botanist track, they give you a cowplant berry.
