Walmart, Academy, and other places usually carry Coghlan's products, one of which is Screen Patch and the other is a complete Nylon Tent Repair Kit. I've seen the products hanging in the camping accessories section.
http://www.coghlans.com/catalogue/pr...t.php?catID=15
The screen patch portion in each is a mylar, self-adhesive type screen patch.
rgds,
WT