[Finally an easy question. ...Sort of.]

All over Kanto and Johto both. Usually in the bigger towns and cities, though. Anyplace with a pokemon gym. I think there's a list somewhere on your gear, too, but pretty much any town you get to, if you just ask anybody where their gym is, they can tell you.

...You might have to fight them first, though. The gym leaders, I mean. Not just the regular people, although some of them, too.