2 Panther platform coil springs (any 1979 to current. Crown Victoria, Grand Marquis or Town Car (pre RAS (1990)) and 2 of the spring isolators between the top of the spring and the perch.
Part number for the isolator is D9AZ-5536-A, need two of these. They are around $10 each.
To maintain a smooth ride you should use the base rate (130lbs/in) CV/GMQ coils and not Police Interceptor/Taxi springs (160lb/in).
You can just order the current production Grand Marquis springs from your local dealer, they will be less expensive than older variations and are a direct fit (the load rating is what changes between year range, meaning the ride height given static vehicle load) and have the lower rate.
If you don't mind, or would prefer a little stiffer ride and better load carrying capability you can use the CVPI springs. Part numbers can be found here p71interceptor.com/rearsprings
The current production springs supersede older ones, so 6W7Z-5560-A (or -B) is what you would order.
I'd also advise against used coil springs for obvious reasons.