I think this is the part you're looking for: MUP7000110 (search on "slide"), but Capt. Rick should confirm this before you order.
Just so you know...I replaced mine last year because the old one was bent. Well, the new one is already bent a little. The design is a bit weak. When the seat is all the way back (as I always keep it), there is no support beneath the rear tracks. People also lean on the seat back, using it for support getting in and out of the boat, which doesn't help the situation.
If yours is bent (which is likely why it doesn't slide any more), you might want to look into buying or fabricating something stronger. Maybe someone has some ideas of where we can source or how we can fabricate a better sliding seat mount??