Go to 3dCommune.com. Cooler made a bishop's hat. I believe it's still in the 3dCommune Free Stuff.

If you're using Poser 5 or 6, check out PoserFashion.net. The Poser 5 section has a free dynamic monk's outfit for Michael 3. Re-textured, it might work for bishop's robes. (You could make a stole yourself with the high-res square in the cloth room.)

If you're using P4 or PP, DAZ has some "wizard robes" for Michael 3 that might work with the right textures.