Posted my experience here: http://www.immigrationportal.com/showthread.php?t=268313
if you are already registered you can print online the proof of registration. Once when your registered they mailed proof of that, I don't know why you don't have proof. The other option is to call them and ask to send you a proof of that.Sashko,
How do you get proof of selective services? Aside from the selective service # which I got from their website, is there anything else you need?