Looking for items 15k or less that protect a character from stunning.
Your best resource for this kind of thing will always be Ernir’s Lists of Necessary Magic Items, the author of which goes by @Ernir around here.
One of the lists he mentions is for Stun negation, because
Why you need it: Stun is one of those conditions that makes you unable to act. Which means that when you're stunned, your character is a sitting duck, and you're sitting at the table counting cheetos.
All of his entries for this fall within your price limit:
Third Eye Clarity (MIC). 3000GP, face slot. Once per day, negate the Stun as an immediate action. Can negate other nasty stuff too. Great pick.
Talisman of Undying Fortitude (MIC). 8000GP, held. Swift action activation, gives you lots of undead immunities for 3 rounds, including stun.
Banner of the Storm's Eye (MIC). 15000GP, held or shoulders slot. Negates stunning and other nasty stuff for you and your allies. Top pick.
Wakeful Mind graft (FoE). 14000GP, slotless. Other minor benefits, including no longer needing to sleep. Also makes you slightly less healthy and less knowledgeable, but that's the price you pay for having robot components embedded in your head.