Hello Brittany, and welcome to the forum!
I cannot speak for Canada, but here below the border the best source is your local school district. Generally not the main school office (though possibly), but the bus shop. We tend to call the yard and shop were school buses are kept and serviced the "bus barn".
I have walked into a few bus barns and always been greeted with friendliness and helpfulness, and even enthusiasm for bus conversions.
One should be neatly dressed and groomed and "behave like an adult", when making such inquiries -- but you knew that.
Many school districts do not actually own and service the buses themselves. Instead, they hire a company to provide this service.
Around here, a huge international company called First Student dominates this market. And First Student is a topic in itself.
But the folks in the bus barn are usually friendly and helpful "bus people" without regard to who signs their paycheck.
About tires. It is common to put near-worn-out tires on a bus that is being sold. If the bus is otherwise good, and you have $2,000 left in the budget for tires, don't get hung up on the worn tires.
Is that a good start?