Seat belt/airbag sensor

I would not leave those disconnected, they signal the system on which seats are occupied and how heavy the person is. That allows the system to regulate the force of the airbags and which ones will deploy. What were the new seats out of? If they have the same type sensors it may be possible to adapt the new seats. If not you may need to replace the sensors in the seats to make the system operate.

They were out of a 2007 Chevy Cobalt SS. I do not want to leave them disconnected. The connection I have is smaller then the one the new seats. Is there an adapter plug I could use to plug them in?

No adapter but if they are from the same year and model look at the wire colors. If those are the same you could likely just re-pin the connectors to the original wiring harness. Basically remove the pins one at a time using the correct tools and switch the outer shell to fit the old harness. If possible take a picture of the two connectors, that should give an idea if it is possible. Then you still need to add the seat heater hardware and wiring.

Where do I find a wiring harness to hook up the heat?

To get the correct connectors and wiring you would need to dig into the wiring your car already has under the seats and carpet. You also need the switches that are part of the heater controls in the dash to make the seat heaters work. If you still have access to the car the seats came from, that would be where I would get the parts from.