Since I answered the last one, I may as well do this one as well. Same rules as the last one, though. That being, this list involves some speculation.
Taillow to Swellow
Episode: That's Just Swellow
Location: Route 119 (?)
Yes its a sad sign when I'm only on the first location, and have already brought out the question marks! Swellow evolves in the anime-exclusive location of Crossgate Town. I am saying Route 119 because a couple of episodes after this one, Ash is at the Weather Institute. It could be Route 118, though, there is no real way to tell.
Treecko to Grovyle
Episode: Exploud and Clear
Location: Route 104
Now before you completely dismiss me for saying it evolved at Route 104, when that is where I suggested Ash caught it in the first place, hear me out. Ash was returning to Petalburg City to fight Norman when his Treecko evolved. 2 episodes later he was in Petalburg. It is unclear which path Ash used to reach Petalburg City, so he could have used Route 103 and Route 102, but earlier episodes seem to point to Route 115 or Route 116, so I'm thinking Ash came down the western shore of Hoenn to return to Petalburg. This logic means Treecko evolved on Route 104.
Grovyle to Sceptile
Episode: Odd Pokemon Out
Location: Somewhere in Kanto
It evolved when Ash was challenging the Frontier Brains in Kanto, so you will be stuck with a Grovyle in Hoenn. I can't even tell exactly where it evolved, but maybe Route 19, since Ash was in Fuschia City just before, and then heading to an Anime-only island afterwards. Either way, it doesn't really matter since it isn't in Hoenn.
Snorunt to Glalie
Episode: Rhapsody in Drew
Location: Slateport City
Ash's Glalie evolved while May was fighting in the Hoenn Grand Festival. The Festival is always held in Slateport City. If you want to get the timing right, then only a few episodes later Ash was in Ever Grande City, so do this just before going to Victory Road.
Sources: List of Pokemon Episodes, pages on Ash's Pokemon, and on the location of Grand Festivals
