@Anitalifegurl Do you have the Get Together EP? If so, go to the retail neighborhood included in the pack called Magnolia Promenade. There is one fishing spot in the neighborhood on the pier. Your Sim will consistently be able to fish up cowplant berries from that spot. If you like to garden, it's a reliable spot for Dragonfruit too! Need some extra simoleons? My Sims also get Voodoo Dolls from that spot all the time and those go for 950 simoleons a pop! :)
Actually, cowberries can be caught fishing at almost all the fishing spots in the game in my experience. But that one spot my Sims always get them, every single time.
Anyway, there's one other spot I've found that has a higher percentage of fishing up cowberries than any other. It's a base game fishing spot too if you don't have the GT EP. Go to the fishing spot way in the back of Magnolia Blossom park in Willow Creek. Have your Sim fish at the middle sign out of three fishing spots available back there. That spot is the one fishing spot to catch rare fish in the park too. Oh, and you can get Dragonfruit from that spot as well. Hope this is helpful!
Happy Simming! <3