I scanned a set of classified ads back to December and didn't find a cougar.
on your string, first I would make sure I got a B50 string and not a fast flight string.
there are a couple of mail order places to get one. http://www.3riversarchery.com/ http://www.kustomkingarchery.com/
recurves will use a 4" shorter string than the length of the bow. longbow 3" shorter (rule of thumb). get them on the phone and tell them what you have and they will get you the right string.
actually valve depends a range of things. condition, rarity, poundage and popularity are a few things. I'm no expert at telling value. I get my info from ads on trad sites. ebay has inflated prices at times.