Need a good source for GM part #'s

[SCOTI]

Does anyone have a good source for obtaining GM part #'s? I've tried getting an HEI style rotor for the 250ci 6cyl in my beater through the dealer & he can't seem to find the correct P/N.

I know A/C Delco #1894979, for a 75 250ci 6cyl w/internal coil, is the correct distributor cap because I saved the box top & info the last time I did a tune-up. I thought I saved the rotor # as well, but apparently didn't put it w/the other stuff.

Since I was going to order a window regulator & have to go through the trouble of paying in advance, I thought I would get tune-up stuff for both trucks while I was ordering.... and since neither truck came w/HEI, they don't know how to find the info. because the books don't tell them whether the P/N is for a distributor w/external or internal style coil.

Any suggestions??????????