Obtaining Mac OS media

I find myself with a newly initialized Hard Disk in my iMac (I won't bore you with the details of how that came to be) and I have misplaced my original DVD. I have a Drive Genius Boot Disk, but it doesn't include an App for the AppStore - is there any way for me to download Mac OS, or am I going to have to go to Apple and request a new DVD?

Thanks in advance.

Neil