I’m not sure. Radar had a FZ600 and sold it recently on here and I think he may have had a spare rear shock. It might be worth dropping him a PM.
Otherwise you could get it reconditioned?
https://www.allbikeengineering.co.uk/html/shocks.html
Failing that when I changed the shock on my Katana I measured the distance between the fixings then went to a bike breaker and found one that fitted. I ended up with one off a Triumph.
Good luck with the search