I'm not surprised that you can't find new ones. Best bet would be to keep an eye on Ebay for second hand. Failing that, Felindre may be able to advise on more modern spools that could be compatible. Plenty of the 3000/4000M spools on ebay now. No 2500s I could find.
Its odd,you`d think so Sam but I have a more modern 2500 super gt and exage 3000m and neither spools quite fit the older super x,the stroke is slightly different(modern spools rubs at the bottom stroke) and the fit isnt right.Felindre dont supply them either.
I know that not all Shimano spools are exactly the same. A very small difference can render them useless. I'd never blindly buy a spool without the advice of Felindre.