Surely this is 100% cotton, and therefore completely natural? Also, I don't see where it says that it is waterproof? I think you would need to paint it. In my experience sewing, anything sold as craft fabric is usually of low quality.
I don't know how much you will need, but this place sells loomstate (basically unbleached and needs washing) and if you buy more than 3 metres the price reduces, and again at 10 metres. Loomstate should be OK if you are going to paint it.
http://www.whaleys-bradford.ltd.uk/fabrics/section-b-cotton-calico-loomstate-cottons-canvas?limit=36