Found a few sites for you...hope they help...
http://www.allfreecrafts.com/sewing/window-curtains.shtml
http://www.denverfabrics.com/pages/static/curtains/free-curtain-patterns-links.htm
http://www.craftandfabriclinks.com/decor/checkedpanels.html
http://www.freepatterns.com/list.html?cat_id=322
(That last site is one of my favorites, tons of patterns for all sorts of crafts and all for free, just have to register. No spam mail from the site either, always a bonus!!)