The seat rails may be rusty and cause the plastic inserts to bind. There is only a small amount of clearence between the upper and lower plastic inserts, any rust in the rail channels will make moving the seat either way will bind and stop. Once you have the seats out, it will become clear what I'm talking about.

I wire brushed and ran sand paper in the rail channels to remove rust, then painted and later lubed the rails. Now they work like new!

Sun Visors for these cars I've found are priced even higher than the Navy's $600 toilet seats, by compare! I seen them listed recently as high as $360 each!

Try this link for used http://www.kkmfg.com/