Check out yard sales and the like.

Many times the two passenger seat gets left behind when the van is traded or folks move.

I paid $10. for my last one at a yard sale.

Junkyard price for the one in the pics was $40.

Nice part is, the seats come out by simply popping a lever.

Fwiw, the seat belts in the middle are attached to the seat by a single large bolt that bolts to the rear of the seat frame proper.
It would be easy to run a reinforcing strap from that bolt to a good anchor on the floor.
Or . . . you could do the usual seat belt install with the belts going to properly reinforced and anchored floor mounts.
I installed an extra crossmember in the Deuce frames on both my roadsters specifically for seat belt mounting and a roll bar lateral.
The crossmember has threaded and stepped weld bungs for the seat belt mounting bolts.
The bungs are installed from inside the frame and welded.
The weld is sturdy and if it failed the step would prevent frame pull-through.

The side - retractable - ChryCo seat belts mount to the factory seat frame so you get those when you get the seat, provided they're still on the seat, most times they are.

A lot of times junkyard vans have the side door left open.
Check the seat for water damage.
An undamaged seat will last a long time as is and since upholstery is one of the last things we do, that makes life easy for a while....