Join Avenue (Antique Shop), Plasma Frigate B2
So you really just have to score at Join Avenue, or trade for a Pokemon holding one.
Magmortar does have higher special attack, but physical moves are still usable. It gets moves like Earthquake and Cross Chop which are all very useful. So you could so for a Lonely, Mild, or Naive Nature. But I really do recommend going all special, with a Timid or Modest Nature.