Are 22 tooth (or smaller) and 32 tooth rings available for Bio-Pace cranks on a mountain bike? I would like 22/32/38 crank rings. All I can find are 28/38/48 or 51/52. A 30 tooth middle ring from a road bike might be an option - if I can ever find one.

Why? You ask? My knees and hips are shot, thanks to a hereditary bone defect.