Bontrager Select was an entry-level Trek saddle in 2007-10. I don't think you need any model numbers. They are not made any longer but you might be able to find one in decent shape on eBay. You just have to make sure to get the right width. Since it's a girl's bike, it's presumably a WSD and the saddle is women's specific (which basically means that it's wider than men's saddles).