Does anyone know where I can buy a rear-ward facing 3rd row seat with belts for a 1990 Buick Century Custom Wagon?

I purchased the car a few months back from a neighbor for $1500 and it has an astonishing 51,000 original miles and it is in mint shape. The shocks, struts and tires have been replaced due to age and rot but other than that all it needs is a fuel pressure sensor and some new plug wires and it's running like a new car. It belonged to his elderly father so he didn't order the seat as an option.

Any other things I should look out for on the car?