The easiest way to replicate it is with leather - but that's not exactly cheap.
There was a guy on the old forum that advertised replacement covers but I don't think he's been heard from for a long while.
Very, very occasionally something like this comes up on ebay
http://cgi.ebay.co.uk/LANCIA-BETA-SPIDER-ZAGATO-INTERNO-SEDILI-FODERA-TESSUTO_W0QQitemZ220555002103QQcmdZViewItemQQptZRicambi_automobili?hash=item335a1a40f7 Other than that carefully take the covers from the passenger seat and ask a good upholsterer to make patterns from them. An interior trim specialist should be able to get pretty close on the colour.
Be a bit careful with Felix at Lancia Cape town, you need to describe what you want very carefully.
And post some pictures !!
Stuart