Where can i buy fabric bolts?
I assume you’ve already tried fabric stores without success. If you can’t get any there, you can use other things to wind your fabric on: a nice piece of PVC pipe (if you want a roll) or just a nice 1 x 4 board (if you want a flat). You can get either for cheap from the hardware store. You could also use polystyrene planks, folded carboard or foamboard, or even bamboo from a garden center!