Stretcher Frame:
For canvas art, an internal frame WITH corner-expansion provisions for future retensioning of the canvas, and a specially-tapered profile to limit the area of contact with the back of the canvas.
Strainer Frame:
For canvas art, an internal frame WITHOUT corner-expansion provisions for future retensioning of the canvas, usually with a specially-tapered profile to limit the area of contact with the back of the canvas.
If the art is not worth stretching properly, why not just mount it to a board, such as GatorFoam, Masonite, or foam center board?
If cheap is the goal for stretching canvas of no long term value, you could buy dimension lumber from a home-improvement store; 1" x 1/2" will work for canvas up to 11"x14", and 1" x 2" will work for canvas up to about 32" x 40". Over that size, perhaps 2 x 4 would be the best choice.
Seriously, if the art is worth stretching, I suggest doing it properly, using wood that has the correct tapered profile. Otherwise, creases or cracks will eventually appear on the face of the art where the canvas flexes against the wood beneath it. That profile may be purchased in length from several distributors. Expansion provisions are recommended to avoid having to completely remove and restretch the art later.
If preservation is an issue, you could line the contact area of the stretcher bars with Lineco rabbet-lining tape; a foil tape backed with paper. You could also paint the wood with a good quality, water-borne polyurethane varnish, but that's more work and a less-effective barrier than the tape.