I've seen some special adjustable driver's seats in shuttle buses and school buses built on E-350, E-450 and Chevy express chassis. I don't know if any of these are spring base, or if they are just more adjustable and possibly better built than the seats RV manufacturers buy.
I've also been in a few shuttles where the driver's seat looks like an OEM piece, so for the companies that make these buses either some don't install special seats or they are an option. But in any case, if shuttle and school bus manufacturers are installing special seats, somebody out there makes them.
Maybe one of the manufacturers of these Type A school buses can give you a lead to a driver's seat supplier. School bus manufacturers include Bluebird (Microbird subsidy), Thomas, Collins, Starcraft, and Transtech. Starcraft is also a leading manufacturer of shuttles, small transit buses, executive transports, party buses and so on, so they are likely to have a lot of connections to interior furnishing suppliers. Starcraft Bus is a Forest River company (one of their five bus companies). Don't look for Starcraft at Thor, they use the name for RVs only (someone else owns it for boats) but Thor may also own a transit bus company or two.
Knoedler makes a suspension seat for the Sprinter, Extreme Low Rider, for about $1700-1800; I don't know if this would adapt to the E-series. This manufacturer shows other models for RV applications, but these might be for Class A motorhomes, which have a different driving position.