Try SMS Auto Fabrics in Oregon. If anybody has it, they should. It won't be cheap.

https://smsautofabrics.com/collections/1971-pontiac/products/1971-pontiac-catalina-4-door-sedan-trim-507-complete-interior

I don't have knowledge of the 1971 trim codes but it could be found in a Master Parts Catalogue for '71 or an original Paint & Interiors booklet that the dealer would give out to customers when deciding how to order a new car.

As for the "59", that is probably referring to the body style, however the only valid ones for a '71 Catalina are: 69 (4-door sedan), 39 (4-door pillarless hardtop), 57 (2-door hardtop) & 67 (convertible).

It might  be worthwhile to see what a local upholstery shop can get you. You might get something reasonably close for a lot cheaper. That generation of full-size models used full foam construction for the seats and it compresses significantly when you sit on it, stressing the upholstery & seams, causing them to split.

Good luck with your project!

Not to hijack Victors post, but is there anyone here in the GTA that can be recommended to sew covers?