The U.S. B&N website has quite a few Nook Touch covers for sale at $15 and less. With their finally refreshing the line - and probably dimensions - these may go soon.
http://www.barnesandnoble.com/s?CAT=...w=grid&sort=SA
Ebay is also a great place to pick up covers especially since the NT has been out so long and manufactures like to refresh lines. I picked up two very good quality Nook covers at $5 each, shipped.