Hi Kate,
My DS loved to be swaddled and he was a solid one...he outgrew his swaddling blankets very quickly. I recall getting three extra large swaddling blankets from a family member. If you're handy with a sewing machine get some flannel from the fabric store and cut it as much larger as the regular sized ones as you like, sew it inside out on three sides, flip it and then blanket stich the open end. If you're not, I bet you could find someone from church or work or somewhere who could do this for you either free or for a minimum charge.
Also, I googled it and this came up. http://www.diapers.com/html/sg/extra-large-swaddle-blanke...
enjoy the swaddles and the snuggles...............they go by so fast!!!!!!!!!! hoping this helps, S. :-)