You would be very lucky to find new or NOS any more, I think. eBay is probably your best bet. 5 minutes searching found these used saloon ones:
https://www.ebay.co.uk/itm/403698028...Bk9SR7ClsfqPYQ
... and these too, from Best of British Rover. Read the full description. They are new but would need some modification to fit:
https://www.ebay.co.uk/itm/263795049...0AAOSwuJ9jaNlv
... but these could be a cheaper solution
muffs.jpg
David