I am not sure of the best online source as I have not found a cheap source as of yet. Carvewright does sell it in the store………..
I think the curved lithos you are seeing are done on 3in pvc pipe and carved on the rotary jig.
PVC is as cheap as you can get for lithos. I buy 20ft for $11 at a plumbing supply. I just take a saw and cut it in half to transport it in my truck.
Brent
A,B machines, PE,probe,conforming vectors, centerline, 2d,3d,stl and dfx imprt, rotary , 3.xx, Pattern Sculptor
OS X Yosemite
Woodwork is what I do to pay for my tool addiction.