I have been going up to the Bayside Harley Davidson to get OEM part numbers for different things I need for the 1990 Sportster. The parts guys at Bayside have been extremely helpful in helping me find OEM part numbers, even though they have not had (or been able to get) much of what I need (most of the stuff is only aftermarket now or NOS on eBay).
Neither the Clymer Manual nor the Harley Davidson Service Manual list the OEM part numbers for things. There is a parts catalog, but it is like $60. Bayside has one so I have just been using theirs. I was thinking about picking one up since having the OEM part number helps when searching for things on the Aftermarket sites or eBay. Then I came across this site by accident, Ronnie’s Harley Davidson OEM Parts Finder. It’s an online microfiche site with the diagrams and the OEM part numbers!
I’ll still probably ride up to Bayside when I need some help (or just to chat), but with this I can locate the OEM part numbers to help when I am searching for something.