Installed many sets of those plastic covers and I would rather wear out the original covers before installing them on a car I owned.
My grandfather put a set on his new 55 olds and again on his new 67 olds. My grandmother HATED them and always said "why should I sit on plastic just so the next guy can have brand new seats?".
Had em on my 57 Chev, got em from a chain of upholstry shops back in the day. The outfit was called SureFit. They were originally marketed by an outfit called Fingerhut Co. Thats all I got...