Most chevy astro vans, dodge caravans, ect have rear seats with integrated seat belts. They also quick release from the floor making it super simple to build a mount for them. Also if you build the same mount the van had on the floor, you can still have the quick release feature in your bus.
I would head down to your local pick and pull. I'm sure you will find what you need at a fraction of the cost of anything new.
"Don't argue with stupid people. They will just drag you down to their level, and beat you up with experience."
Patently waiting for the apocalypses to level the playing field in this physiological game of life commonly known as Civilization