I would telephone these folks (be sure you are sitting down)
http://www.lebaronbonney.com/factory-replacement-seat-covers.htm
I used them a few times, when they were Autocraft. One was to recover seats in a 1975 Dodge Colt. For short money they made me a set of front and rear covers that actually were *better* than the original upholstery. Frankly, I was floored. Another was restoration seat covers for a 1969 AMC Rebel. That car was a driver; my dad got it for me and did some repairs; I drove it thru high school and college. When I graduated I bought a new Mercury Lynx and gave the Rebel back; my mom and dad drove it off and on for a few years; they then sold it to a person who wanted it to restore. What sold the car? The interior. Dad said the buyers were giddy about the interior. He did not tell them it was replacement Autocraft upholstery. The car was brown metallic with a gold metallic roof and white Naugahyde upholstery. They matched the original upholstery color dead nuts on and used seam piping in metallic brown to match the exterior paint. Again, for fairly short money.
The Riv interior is leather and uses different stiffness of upholstery foam in its different parts. Probably one of the most comfy interiors ever to come from Lear, for any car. Again I would be sitting down for this one but yeah, I'd call LeBaron Bonney and ask about these. Their .pdf shows the '95 - 99 Riv covers at $1200 - $1,500 for the EZ-Boy kits. Does not show what those are made of - leather or correct color vinyl, (or maybe ultrasuede?) and if they know the different densities for the foam in case you have to replace any.
Bottom line for me is they would be my "go to" safe choice but again calm down before you call them and you'd better really want to seat reupholstered because the price is pretty lofty. That said, what you get from them for that money should be correct down to the pockets on the seat backs and the method of attachment (combination of hog rings, straps, zippers, or what have you).
