I think you can find one at Roland's Cabin east of Imperial City, just north of the road to Cheydinhall. I accidently nicked it off his table when I was there =)
Stop! You've violated the law! Pay the court a fine or serve your sentence. Your stolen goods are now forfeit.
... on a less nerdy "oh my God I've memorized the guard speech"-note, I've found more than a few gems for sale at stores (Jensine's and Thoronir's, at least) and in Mages Guilds. Some houses also have them as decoration, if you're not too opposed to stealing... Otherwise, you can find them in random loot.