Lost restore DVDs

Simple answer- you don't.

More complete/helpful answer is to call Apple and request a set of "fulfillment disks" which is what they call the restore disk set that shipped with your computer. They will need the serial number of your Mac to verify they are shipping the correct disk set and will charge you a moderate fee. A recent set of two disks that I ordered was $32 shipped and only took about 4 days to arrive in the mail.