You should have a plate or sticker similar to this somewhere on your bus, usually somewhere near the driver's seat.
On mine it's above the window next to the driver's seat.
The plate, or one of the plates if you have more than one, like mine, will be an entry labelled "INC VEHICLE MFD BY:" which of course will be followed by the name of the manufacturer of the chassis or
'incomplete vehicle'.
In the right photo above it's the second entry on the left side pf the plate - you can see my 'incomplete vehicle' was manufacted by Thomas, so they built the thing from the ground up.