The lowest level option is the 4th level class feature of the Spirit Shaman class. The cheapest continuous protection is the Dracanite Helm for 12,000 gp. The most ironic option is to be a level 11 binder, figure out how not to sleep, and continuously summon pseudonatural lantern archons.
Expensive option: Buy a "Dracanite Helm" (Ghostwalk p70, 12000 gp) which: "... protects the wearer from all forms of possession." (and gives +2 AC)
Cheap option: Buy the "spiritwarder" shortsword from Ghostwalk (p66). It has protection from possession 1/day (which explicitly blocks possession, not simply suppresses it), and it automatically triggers on a possession attempt. At 4500 gp, you can get a golf bag of these if absolutely necessary.
Very cheap option, some downsides: Become a necropolitan (Libris Mortis p114). The ritual delevels you from 3 to 2, but is otherwise cheap at 3000gp and 1000xp if you do it exactly when you hit level 3). Surviving as an undead PC is left as an exercise to the reader, but it can work quite well in the right campaign.
Feat option: You can get a Lantern Archon or Coure Eladrin (looking at the familiar handbook) with the Celestial Familiar feat, a way of getting a familiar in the first place. These familiars are featured in the planar handbook and BoED. You can also use Planar Familiar via planar handbook to get a lantern archon if BoED isn't allowed. Alignment restrictions apply.
Very expensive, aggressive, evil option: Carry a darkskull. Moving unhallow means that your undead are awesome, and you immediately suppress all mental domination within range (due to the mobile magic circle against good.) .. From a close reading of protection from evil, the controller/controlled creature doesn't seem to get a will save to resist that part of the magic circle. Useful if you're on a quest to "liberate" other Inspired. (And being an evil cleric will help quite significantly, since your army of undead will both be helped by the spell and immune to mind control.)
Be a cleric. Divine metamagic extend persist protection from possession. (It'll last for 48 hours, recast it every day.0
Incantrix/spelldancer extend/persist prot possession as wizard.
Spirit shaman level 4, infinite duration protection "from spirits." Very apposite class for the character. Make sure to look at the handbook.
If you can figure out how to not sleep, maintaining a summoned lantern archon via the binder's Zceryll bind can keep protections up.