I've tried a couple of places. My best experience by far was Canvas On Demand. They're constantly running Groupons - google 'national Groupon list' or something like that and you should find one pretty fast. Great customer service and they'll replace it if you don't like it (I've done that).
Try to set up a pro account (
http://www.canvasondemand.com/professional/)- you'll get better prices (even while using a Groupon), and your first 16x20 is free (+$15 or so shipping). You must have some sort of photography website to qualify (PM me for a possible workaround).
I've used Adorama too and been very happy, but Canvas On Demand has a nicer final product for cheaper. And I've had bad experience with Artscow and Costco - lousy quality and colors, and it started to flake after a couple of months.