You don't really have a lot of options, sadly. Mints are the best way to go about this, but they're exclusive to SwSh, which, even if you had those games, Poipole isn't even coded in there as of writing this, so that's effectively a dead end. You're also too far in the game for soft resetting to be an option either, and since Poipole is a legendary/legendary-adjacent 'mon, you can't breed for a new one. Really, your only options are to attempt to trade for one and hope for the best, or start a new game, unfortunately.
Hope this helped!