Most unique spells can't be sold by vendors but have 2 - 3 locations to be found, like the spell that turns iron into silver and silver into gold, so I was thinking why not have some more elidgible places?

Here's a list of places I though of:

Hag Rock - Top Area near Mehrunes' Pommel Stone

Illinalta's Deep - Main dungeon

Scalleum of Boethiah or Ansilvund - Somewhere on the offering shrine to Boethiah or deep within Ansilvund

Castle Volihkar Secret Wing (DG) - The wing uncovered in Valerika's garden

Raven Rock Mine (DB) - Deep within the mine