Peter Gunzl used to have a lot of Westfalia bits including seat covers, I had some California Isri ones off him. It might be worth checking what he's got left, if anything.
E D I T. Peter also does a fair number of different fabrics, by the metre.
There's a company in the Netherlands - West Trading BV - that has some of the original VW upholstery fabrics by the roll, but I think they may be trade-only. I guess if you're dead serious you might be able to get friendly with an upholster and see if its feasible to get a roll ordered.
E D I T: I just checked on their website, and they are listing fewer original T3 fabrics than they had a few years ago. Gunzl may be a better bet.
HTH
PS. The seats are basically ambidextrous, you could always swap them over and give the passenger the tatty one ! (Might be good to do this 1/2 way through their life to even up the wear..)
PPS. Another option is to keep an eye out for old rear seat/bed covers in the required fabric at shows etc (when we have shows again) to use as fabric donors to refurbish the front seat covers. For my Isris had to do that for the lower covers, it worked out fine.
PPPS. Also, it might be worth getting in touch with Frank Grote (
https://www.vwbusparts.com/) he knows his Westies...