Hey all the above listed are great sources for parts! Luke, C & J and Caylor's are all very busnut friendly and support our needs with more than parts in the form of knowledge and etc.
But lets not forget people like John @ Central FL Bus Repair, Nick @ Nimco or Dean @ ABC, and other places that sell SOB's (some other brands) they all have a wealth of info and usually a pretty good stock of used parts too!
As for MCI just call them they have people paid to look up what you need! (I do it all the time!
and I have the parts books and a customer #, I just like to talk to a person not a machine or have to hope the part I'm ordering online is the right one! As for parts I have to order more than once I keep a list of past parts bought and do order those online!)