Thomas won't sell you drums or tires. They don't make either, as they buy the stripped chassis from other truck manufacturers as stated above.
If you've got a Mercedes engine in your bus, it is highly likely a Freightliner chassis.
Find a Freightliner dealer if you need drivetrain parts that can't be sourced from aftermarket shops.
With a little research you should be able to buy both items local to you or online, though.