Joannes and Hobby Lobby usually have canvas and 500D nylon in their bulk fabric selection. Can get expensive by the yard so wait for their sales/coupons.Where is a good place to buy the heavier fabrics? I am in Austin. Are local shops likely to have this stuff? When I took my machine in the local shop, I didn't see much besides garment and quilting fabrics.
Look up auto interior repair, tent/awning makers or boat cover makers in Austin. If they have a store front, they may have a selection of heavy canvas or vinyl they will sell by the yard. Also, if there is a large textile repair/manufacturing indistry in Austin there will be industrial fabric supply companies around town.
Also, if you have a Cabelas or Bass Pro, and they have bargain barn return center They will sell returned tents, pop up tent covers, canvas tarps, etc.....at deep discounts and I've reuse the canvas and zippers before.
Lastly, if you want heavy duty waxed canvas, simply go to the local hardware farm and ranch store and buy pre-made waxed tarps or painters drop cloths and cut them up. This can actually be alot cheaper by the yard then ordering it on-line.
I now buy canvas and Cordura nylon in 2000+ yard quantities (60 to 100 yards per roll) monthly by the pallet but those supply companies normally won't sell remnant or cut rolls so you'd have to buy a minimum of 60-100 yards plus shipping.
I will sometimes have "drops" of leftover materials that I store up for small projects and normally would be happy to let you have some but, since moving into our design shop I got rid of alot of it.