Because wearing armor reduces spell effectiveness, it's recommended to wear robes and other mage-orientated gear when playing a pure spellcaster in Oblivion. I've played for hours now and have only discovered basic robes with no abilities. The magical merchant items sell for thousands while my gold supplies are in low hundreds.
Are the any recommended quests with suitable rewards or some other specific locations?
You don't have any option besides looking for generic clothing and enchanting them yourself. This is the most effective way to get the best possible equipment for you.
Refer to the clothing list for choice items. Unlike Morrowind, every item has the same enchant value (I.e. you can't put better enchantments on an archmage robe than on an apprentice robe), so decide by item style. For info on how enchanting works, check the uesp wiki.
However, I wouldn't say the reduced spell effectiveness is a big factor, you only lose 5% effectiveness once you reach 50 in the armor skills and armor generally can hold stronger enchantments than clothing and protects you much better (you pretty much have to spend all your enchants on armor when wearing clothing or have to constantly keep a powerful shield spell up). Only go for clothing, if you want to do it for roleplaying or challenge reasons. It is much more optimal to wear armor.
Progressing in the Mages Guild questline is recommended since you gain spells and enhanced items as rewards — completing the Join the Mages Guild quest gives you Robe of the Apprentice and you gain a custom staff in the following quest, with more prices to follow.
