If all you need is a better Will save, you might check whether your DM will allow a custom magic item. For a bonus to a single save, the price guideline is (bonus squared x 250 gp). That is, a +2 item would cost 1000 gp, and a +5 item would cost 6250 gp.
If you don't know a friendly spellcaster who could craft that for you, or if your DM does not allow custom items (hi dcollins!), you can shop for a Cloak of Resistance. It gives a bonus to all saves, but is more expensive, at (bonus squared x 1000 gp). At +5 it costs 25,000 gp.
You'd also get some benefit from a Periapt of Wisdom or equivalent. This would be even more expensive-- 36,000 gp will get you a net +3 to your Will save-- but it'll also help with Wisdom-based skills, like Spot and Listen.
In the short term, when fighting enemies who force you to make Will saves, try to get some help from your spellcaster allies. Protection from alignment gives a +2 bonus, and prevents mind control; owl's wisdom increases the relevant ability score; even Resistance can be useful in a pinch.