Kwik Sew has a leggings pattern--or you could self-draft like KitschyCoo described. I did that and was fairly pleased with the results. . .I flipped some leggings inside-out, tucked one leg into the other, laid it flat and traced around it--then flipped it over, placed it along the edge of the shape I'd already traced, and traced the other side. The front and back aren't quite symmetrical, so I found this gave me a better shape than tracing just one side of the pant leg and then cutting the fabric on the fold. (Did that make any sense at all? )
Fabric.com had a bunch of cotton/poly striped knits on sale a few weeks ago. . .I've also seen some on Ebay. Crybaby's Boutique, The Fabric Fairy, and Sewzanne's Fabrics might also be good places to check.