They frequently stock small stretched canvases (usually 5x7 or 8x10) for just a few dollars.
You can buy the absolute cheapest, lowest-grade canvas available and make it feel premium by applying a few smooth coats of your own acrylic gesso. where to buy cheap stretched canvas
If you are just practicing, priming experiments, or working with kids, discount stores offer unbeatable flat-rate pricing. They frequently stock small stretched canvases (usually 5x7