The thieves' guild quests are optional.
Kistandalius in Ars Magicana, the dwarven jeweler in the Dark Forest and maybe Corinna in Verdistis would be good places to check for rings/amulets with either a Pickpocket or Lockpick bonus.
Quick save before talking to / running your mouse cursor over a merchant or hitting the alt key. If they do not have what you are looking for, quick load. After you talk/trade with someone, it takes awhile (maybe 10 or 15 minutes real time) for their inventories to be reset.