You must have the two generations old pointy levers. It is VERY difficult to find spare parts for those now (but if you could I would bet you'd be able to fix it). Start looking on ebay. My guess is that you'll need the lever ratchet and G-springs at least. But if you need much more than that, then it may not be woth your while. Occasionally a whole lever (or lever set) from that generation does pop up so keep a look out.
If you want to convert to 9 speed you'll need to be careful to keep with the pointy levers. The nine speed cable ratio (and hence derailleur geometry) changed between the pointy levers and the next generation ergolevers. Alternatively you could switch to the next generation (1998-2008) levers (in 9 speed) and get a rear derailleur to suit. No nine speed lever will work with your eight speed cassette, in either case you'd need a 9 speed one.
But if you were to go to that trouble you might as well get 10 speed levers and derailleurs (front and rear), chain and cassette to get your grouppo up to date.