Dark types: In magma gang alleyway u can find Stunky, Poochyena, while in aqua gang alleyway u can find alolan Meowth and alolan Ratata. In Obsidia slums u can find Pancham.
Ghost types: Pumpkaboo in Beryl cemetery, if u have a soul candle and u use it on a pumpkin.
I would recomand Stunktank, since learns by move tutor flamtrower, lv 31 night slash and lv 38 sucker punch, but alolan Raticate it's good too if used with hustie, it's just a little risky.