You're not looking for a smaller seat, you're looking for a seat with a thinner back. If you have plenty of headroom, first try unbolting the seat base from the floor and put some 2x4 lumber under it to see if just raising the seat with solve your problem. If it does, then replace the lumber with steel spacer material. If you need a thinner seat back,
these guys may have an option that works for you but it will be more than $2K for a seat and seat mount. You can also consider Recaro seats that start around $1K but they may not have a mount for an E-Van.