Where can you find welkyn stones on oblivion

The best place to look is in Ayeleid ruins, since they're usually full of them. They're also scattered around mages guilds and the Arcane University, as well as in magic shops. Some houses have them lying around too, although I wouldn't recommend robbing houses to try and find Welkyn stones.
Note: If you want to know this because you're doing that quest where you have to find a load of stones for the woman in the Arcane University, you should probably just loot a few Ayeleid ruins and you'll get them quickly enough.