Hello I am new to this forum, and have been on IMCDB for about 3 years. I am also a member of the Canadian Public Transit Discussion Board, which is a board that talks about transit agencies in both Canada and the US. The board actually has a wiki of all the various buses and coaches used by transit authorities, and is usually up to date.

http://www.cptdb.ca/wiki/index.php?title=Main_Page

Example: Greyhound Roster
http://www.cptdb.ca/wiki/index.php?title=Greyhound_USA