My work is better than Walmart, Costco, etc., and I produce my own up to 40x70, stretched or mounted. However, I just had to farm out a 60x80 since neither of my printers could go that large. Installed it tonight and we were all raving about the quality. Best of all, it took little effort over DIY and all we had to do was pick it up and hang it. Worth the cost.
Most of the online canvas should be good up to 40x50. Larger gets more specialized.
That said, some of the pro labs on line will give you very good quality. On the west coast, WCI in Oakhurst or Luxe Editions near Sacto will do you well and are reasonable in price for the quality. Coastal Giclee is another good printer for even large wraps.
If you are getting smaller canvas, mail order won't be to bad for shipping, but if larger, find one close to home to save on shipping issues to your studio.
Bottom line, go for the best quality prints and leave the cheap labs to the snap shooters.