Please Help Me - Posting for my 70 year old grandfather

My grandfather is trying to find the best place to buy seat covers for his 1970 Plymouth Duster (340).

Best answer is to have a good local upholstery shop make them. The kind you buy ready-made, from whatever vendor, is just never going to be as good in terms of quality, fit, durability, etc. Especially not since the market is dominated by Chinese con artists (and American con artists, too).

Read this and this about how real/good auto upholstery work is negotiated and done.