OP, from what I have read, a pre-2001 9 speed should work with your setup.
You should probably research, by pictures or model number what late 90’s models look like and go on eBay and get another derailleur.
As already mentioned you can ask in the Classic & Vintage subforum about replacement rear derailleurs or about rebuilding yours. Unfortunately you need 10 posts to be able to post pictures.
If you are able to rebuild, you might want to still pickup a used derailleur and then wait for suitable parts only one to pop up.
Good luck.
John