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.
Nat