I despair when I go into art shops and find how expensive good quality canvases are. You can get canvas relatively cheaply, but the quality is flimsy and they always have very thin wooden frames and pre-primed with this stuff that just feels fake. At art college we got taught how to make our own canvases (I feel like the plural should be canvii?), but unfortunately I no longer have access to a workshop with a saw, pre-cut wood, weird corrugated nails, reams and reams of free canvas fabric and a burly workshop assistant to hammer for me.
However I have found a cheap solution! IKEA! Ikea has several billion different kinds of plain photo frames (that get really big e.g. 70cm by 100cm) that are wooden, thick and cheap!
Take out all the innards like the perspex and backing board until you are left with just the frame. Buy some calico (this is the cheapest material you can buy. You can go for canvas or linen if you want, but it won’t make a huge difference) from a fabric store. Typically £2 to £3 per metre. You’ll want it about 30cm longer and wider than the frame.
Finally, with a staple gun, staple the canvas as tightly as possible onto the back of the frame. Start with a staple in the middle on the longest side, then the opposite side pulling the fabric as hard as you can. Do the opposite middle staples and then staple bit by bit around the frame pulling as hard as possible. Et voila, a canvas exactly how you want it! Now paint over the fabric with white wall primer. In total making my canvas cost me £22, whereas buying one this size is £42.