Canvases in general are extremely expensive (for no reason), so the best way to save money is to plan buying canvases around sales. In the US, Michaels is where I get my canvases from. If you’re a beginner, level numbers do not matter. I only started to care recently about level numbers because thicker canvases look more professional and I am primarily interested in selling my art in professional galleries.
I will say that Canvas Panels are MUCH better than canvases for a number of reasons, click here to learn more.
Michaels typically has a sale every other month, from Buy One Get One Free, to 70% off. Never buy a full priced canvas from Michaels. You can also find cheaper canvases at Walmart, or the thrift store. You can find some very unique sized canvases at thrift stores and it is undoubtedly the best place to find oversized canvases for an affordable price.
Dickblick is also an amazing place to get good canvases especially in bulk!