I absolutely second Kolorado's suggestion. Bed sheets are probably the cheapest way to get large amounts of material, and if you need large rectangles, they're already cut and hemmed, eliminating most of the time consuming, boring bits of sewing.
My apartment has at least 12 foot ceilings in the front room and the top of the windows are pretty close to that. Even one bed sheet wasn't enough - I actually got three bed sheets for my two windows. I cut the third one in half and sewed the pieces to the bottom of the first two (we're calling this "color blocking." It's all the rage, because I'm sure you're concerned :) ) If your windows are more modestly sized or you're cool with the curtains just covering the window, not reaching the floor, you might even be able to get away with just the one sheet per window. In that case, you don't need to know how to sew. There's one end of a bed sheet that's folded over to look nice and if you carefully open that up with a seam ripper or just a knife, you have a tube to insert the curtain rod in. If you do need the longer length, I suggest just running with it and going for another color, since it will be obvious that you had to add to the main part of the curtain, one way or the other.
All told, my two, 11 foot panels cost about $15. Flat twin bed sheets at Walmart are $5 each and I needed three. Given how much curtain panels that length cost, you could just about justify a sewing machine if you were contemplating it with this project alone.
