The first place I would check is your local sign shop, t-shirt printer. I have experience in this field and can tell you that they buy their material in rolls and stretch their own screens. Even better news is that while the screens are reusable (depending on method used to burn them) they wear out, get pinholes, or clogs and the guys throw it away. You might get some old stuff from them, or talk them into selling you some new. Just give them some story about you want to screenprint a few shirts for a punk show, or benefit or something. Try and stress that image reproduction is not that important (simple 2 or 3 text or numerical characters) and you only need 5 shirts or so, that may work when they ask what it is for, and will most likely keep them from trying to make a buck of you (silkscreening is not cost effective on small runs).
Good Luck!