That grinding is looking a little futile right now, because the game is giving away a bunch of Sinnoh Stones this Community Day. February Community Day features Swinub, an admittedly unremarkable pig sort of thing that evolves first into Piloswine and then into Mammoswine. It's only going to be available during Community Day, and it will require a Sinnoh Stone to evolve. During your region's Community Day, you will be guaranteed 5 Sinnoh Stones. Regardless, you only need 1 Sinnoh Stone to get your Mammoswine, or maybe two if you want one regular and one shiny.

Source: Forbes February 16, 2019 12:33 UTC