If you mean the actual roller itself rather than the whole assembely, I sugest you check out the cost of a small ehgineering/machinist shop turning one up out of a piece of nylon or tuffnol, could be a lot cheaper.
Like Mike Harvey says - get one made by an engineering workshop. If you can get the axle out - no problems. If the axle is fixed you need to fit one in two halves. You can use Tuffnol or Polypropelene or Stainless steel. They basically turn the size you want ideally TWICE then cut each half a little more than half to make a perfect cylinder. The two halves can be screwed (and glued) with four small screws into threaded holes countersunk to keep out of the way of the surface.