The V90 of that year came either with Nivomats or gas/hydraulic shocks in the rear. It is not likely the leaf spring is the issue, but a worn out shock.
There are Bilstein and Boge gas shocks available aftermarket for the shock set-up, but if it has Nivomats, you really have little choice but to get OEM Nivo replacements, which there are many different ones that Volvo put in there. Get the chassis number and go to a Volvo parts counter to find which one's you have, but don't be shocked at the price of the dealer shocks, because you can get tehm for 1/2 the price elsewhere. The list for the Nivo's could be from $400-700 each. WHen I had a '96 960 I found Boge shocks for the rear (not Nivo) for $68, but warn you the ride will not be anything as nice as the OEM Volvo shocks. This is the same for the front struts especially, and cannot be found outside of a Volvo box.