Regular copy paper or notebook paper work for foundation. I make a master square whatever size block I want (usually 7-9 inches) ans draw the lines either diagonally or vertically, depending on pattern. Use black ink or sharpie to make easily visible lines. Then print a bunch of copies, saving the master. String blocks don’t require precise cutting with this method so it saves work and thought.
If you do this, make one copy first and then sew a test block to make sure you like your spacing. And use a short stitch length like 1.8 to make tearing paper off easier.