Onlinefabricstore.net. An assortment of cotton ticking, canvas duck, linen. And they will sell you a 6x6 inch sample for a buck if you want to put calipers on it before committing to several yards of the stuff,
Not ticking very often, though I have found it, but you could check thrift/resale stores for usable patching.
You get a few odd looks, picking through w/a micrometer in hand, but I find good stock there. The best stock is usually on the women's side.