I would try Southbound Motor Trimmers in England-email
southbound@ukgateway.net. They frequently show seats recovered in tartan material in their 911 & Porsche World ads. I believe the predominantly red tartan plaid was called "MacLaughlan" vs. Black Watch (black/blue/green ) and Dress MacKenzie (brown/beige).
My SC has a love it/hate it Cork & Dress MacKenzie interior.