In Raimon City, btw. Now, in the apartment building just to the west of the Battle Subway, go up to the second floor and talk to the guy up there. He'll give you the valuable Sun Stone, which can be used to evolve Monmen or Churine to their evolved form — very handy if you happened to have been using either on your team!
One other very important item you need to pick up is in the apartment building right next door, farther to the west of the Battle Subway. Talk to the blue haired dude inside and he'll give you HM04 (Strength), which is a Physical-based Normal-type 80-power move that isn't a terrible HM, but since you can't get rid of it right away, I would avoid teaching it to any of your Pokemon just yet. Still, though, you'll need this in order to move boulders.