Here's a few pieces of gear and where to get them while leveling your priest or other caster class. Of course if you can get someone to run your through the low level instances, go as soon as you can.

0-10 Edit

Whatever greys you can find.

11-16 Edit

Whatever greens you can find, keep +healing gear it can really increase your value while grouping

17-20 Edit

The Deadmines Edit

Complete the VanCleef quest line (Alliance only), it's long and sends you several places but you will receive [Staff of Westfall]

24-27 Edit

Blackfathom Deeps Edit

Shadowfang KeepEdit