Buy them at action house, there is not such thing as a best spot to farm fillets. And which fillets are we talking about, there are 4 different kinds.

Russia: r. Pra. At the little moon shaped pond off many of the docks. Pearl barely on float rods, 1m depth, 11 to 6 size hook. No groundbait needed. Just a basic float rod and line that can handle 4 pounds. Catch enough filets in 1-2 hours to fill 1-2 days of orders.