Not having much finding where to purchase new replacement seat covers to fit 1954 Pontiac Chieftain.

Would '54 Chevy seat covers fit?

Generally speaking yes, they should fit. However, the pattern may not match what you have.

Have you talked to a local reputable automotive upholsterer? Granted it will be more expensive than buying a re-pop pattern but they may know if certain materials and patterns are still commercially available.

Have been in touch with SMS Auto Fabrics and they supply original fabric, they quoted $1695 for seat covers and $995 for matching door cards.

The only draw back is I have to send them old covers for them to match, I am slightly worried in case seat original covers go missing in on route to them, so researching upholsterers based in the UK.

Just a thought would 54 Chevi door cards also fit my 54 Chieftain.

They will be close but I wouldn't expect a 100% perfect fit.