Back-up with lost Snow Leopard disk

I want to back-up my entire hard drive before installing a new hard drive, but I can't find the Snow Leopard disk I bought a couple months ago. Can I just buy a new one in order to clone the hard drive, or is there some reason I need the original disk?

thanks--

The easiest thing to do would be to buy a USB to SATA/PATA adapter like this one:

Newegg.com - VANTEC CB-ISATAU2 SATA/IDE to USB 2.0 Adapter - Adapters & Gender Changers

Then, use SuperDuper! to make a clone of your system from the old drive to the new drive. Install the drive and you're done.