LEM has sheep casings in 5 oz. bags. $14.99. I've never used their sheep casings but I've done the smaller bags of hog casings and never had any problems. Sometimes they do have some shorter lengths (like a foot or so) in the small bags, but I don't find it to be much of a problem.
http://www.lemproducts.com/product/3733/Natural_Casings