This where I got a big bag for Logan. He only weighs 9 lbs, but he is 15" long and 10" tall. They don't have the one I got anymore. The one I got was a Gigi Olivia large, but they do have some that might work for you.
http://store.doubledbrands.com/lapetca.html
I also found this website that has really cheap bags.
http://pawswholesale.zoovy.com/category/003/