When I did my conversion I needed a 3rd seat. A single RIB seat was too wide for my layout (and expensive). The best VW Oem seat I could find on eBay etc was £800. I eventually bought a new seat from a company called Jennings Seats which was a straight replacement for for the VW passenger captains seat. At the time it was £380 from memory but will probably have gone up since then what hasn’t.