You may try a junk yard.. that's the most obvious.
Otherwise you can call any Blue Bird dealership really. Depending on your area, it's likely there's a Blue Bird dealership that may be able to get you one. If you're actually in the Vegas area try A-Z Bus Sales (http://www.a-zbusparts.com/
) If not, Midwest Transit (http://www.midwesttransit.com/Parts/Default.asp
)has a large parts department, though they sell IC products, but they might be able to help you out.
By the way, It's a 99.9% guarantee your bus does have a 5.9 Cummins, you can remove the "I think"