I trhink you may have good luck looking at trials/unicycle rims. They usually still have braking surfaces, and are FAT.
I recomend kris holmes from experiance.
http://www.unicycle.com/unicycle-hardware/rims-and-custom-wheelsets/kris-holm-29-inch-freeride-rim.html
@Smallfurry started
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I trhink you may have good luck looking at trials/unicycle rims. They usually still have braking surfaces, and are FAT.
I recomend kris holmes from experiance.
http://www.unicycle.com/unicycle-hardware/rims-and-custom-wheelsets/kris-holm-29-inch-freeride-rim.html