First, learn how to make your own strings. It really isn't hard. I learned from a DVD called "Doin' the Twist", but I'm sure there are online resources. Buy some string material (I bought some D97 on close-out years ago and haven't come close to using it all yet), serving material, make a flemish string jig out of a scrap board and you're ready to go.
Barring that, I'd get a spool of thin serving material, carefully cut off the existing serving and just re-do it. Mine always come out a bit small, so I just put a couple of wraps of flat dental floss to build it up and it's perfect.