Visit
http://www.westernhillshonda.com or
http://www.cmsnl.com. Both these sites allow you to check for the availability of a specific part # (91250-430-000 for instance) or just enter the middle part (430 for a 1978-1979 CR250R) and you will get a list of all parts for that model code (bike). Be aware that in many cases parts are used on multiple bikes and the middle code reflects only the original bike. Also things like fasteners have their own numbering scheme independent of the bike. The latter site lists model codes.