I just did seat covers for my back, i needed two sets cuz the placements of the seat belts made it so i had to cut the fabric to get it to work right. Instead i just got two identicle covers. I shopped around everywhere for covers, Walmart, autozone, pepboys, kragen. They all sell the same stuff. For your application, rear with center thingy, you have to look up the style code (seat style) in the manufactuer book they have in the store and then order it, that basically makes it a custom order ie expensive. I bought two "Truck Bench Seat Cover"* each about 45 bucks from walmart. The truck bench cover and our back seat are both about the same length, 56 inches.
1999-2007: ___1988 Bonneville LE LN3 (Vin C) @ 273,000 miles
and still going strong (<-Faded sun fried grayish ie needs paint)
2007-Present: ___1993 Bonneville SE L27 @ 210,000
Speeding around So Cal (<-Beautiful Forest Green)
Mods: Sylvania Silver Star headlamps. Remote start system with pageing device. K&N air filter. Tranny Cooler. High-flow cat with Magnaflow muffler. Full Class II hitch setup.