Hey buddy! I have seen some of these in car performance catalogs like Summit and Jegs or off-road catalogs like 4 Wheel Parts Wholesalers or Dick Cepek. They are listed as a roll bar pads, but they may be available in the diameter to fit your rails.
I have also seen these home-made. The padding was pipe insulation from the hardware store and the covers were simply pieces of vinyl or fabric with the edges sewn and velcro added.
One last shot might be a bicycle shop that sells BMX type bikes or a motorcycle shop that sells dirt bikes. They both use these wraps on the handlebars.
I don't know if any of this helps, but it is all that I could come up with. I have some web sites on my computer at home that may have something. I'll look through them when I get home tonight and post any that may help.
Lynch Mob (F2-3)