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The best place I think is ebay... I will pm you with the name of a user who has some supercab stuff on there right now. What color is your interior? Its not the ultra rare orange is it???
I have found carpet for supercabs by searching for automotive carpet on the net and then just checking each place to see if they offer it. You might want to check Ken's store here first to make sure it is not available from Motor Haven.
You can try a Ford dealer to see if they may still have a neutral color headliner or part number for it. I think you may have to find some material you like and make it yourself.
Interior panals can be painted to mach your current colors. You can also consider changing interior colors to one that was more popular and has more original parts still around for sale.
@ oldcarnut - It didn't cross my mind that standard cab trucks wouldn't have as much need for the split bench. I havn't seen split covers at the common after market parts suppliers. I wonder if some of the more affordable pre-made / general fit Saddleman covers would fit?