Most conversion vans have after market "Captains" seats. I sat in a Pro Master and there should be enough space between the driver's seat and the door to accommodate a seat with armrests on both sides. Flexsteel makes seats for vans as do other manufacturers.
I put one in my cargo van, found it in the 'Scratch & Dent' section of someplace in Indiana, where they do a lot of conversion van work. The seats fit on the base already in the van, not sure about a Sprinter. They were kind of surprised that I wanted to pick it up instead of shipping it, but it saved a few bucks, they're not real light. Google 'conversion vans', there's a bunch in Indiana.
I have an OregonAero seat cushion, too, and think they're overrated, myself, but to each his own.