We bought our van with a double folding passenger and looked for a similar swap but ordinary passenger seats were bringing stupid money never mind the captains versions (had researched this on e-bay and the seat prices didn't look too bad until the van was ours - maybe the seller had loads of bogus seats on there and withdrew them all like everyone seems to do with Range Rover wheels I'm watching!)
We met Richard from http://kernowtransporters.com/ at Camperjam and he did us a great deal on a seat, retro fitting armrests, and fitting the seat to the van while we had a beer!
Captains chair, "safe" base (ie, lockable) and swivel plate all fitted while I waited. Might be worth getting in touch if he's not got any listed and see if he'll do a deal on what you need. Based in Shropshire (IIRC).