I agree you should start at volume 1. I haven't watched the anime but I don't see how you can feel you got the complete picture without reading the mangaka's original vision. Anyway, that aside, the Slam Dunk! manga was licensed by Raijin Comics, but Raijin seems to have fallen apart after only a few volumes, leaving SD in limbo. The scanslations for Slam Dunk are done (the last volume came out very recently) but since it's licensed I can't tell you where to look for it. Just keep looking and hoping.
