Here is a link to the very page of the GM Performance parts site selling the "143" cam:
143 cam page
You can either order it online from GMPP off the above page, or go to your local Chevrolet dealer and order it there, or call Crane and get it from there...whichever you choose, you will end up with the same cam.
Just curious, any specific reason you are needing an EXACT "143" cam? I've got a couple of "original" 143 cams (already broken in![img]/ubbthreads/images/graemlins/naughty.gif[/img]) that I would GLADLY put in a GM box and sell to you!