How to get install DVD for pre-2011 iMAC?

If the iMac was updated from Snow Leopard to Lion or later, then you should be able to erase the internal hard drive using the Disk Utility in macOS Recovery. see > About macOS Recovery - Apple Support

Then just follow the instructions at > How to erase a disk for Mac - Apple Support

If it was never updated beyond Snow Leopard, then you to make an installer on a flash drive or a clone on an external drive, startup from it and then use Disk Utility to erase the iMac's internal drive.

What system version is it now running?

Sometimes Apple can burn duplicate OS install/restore disks for older models. NOT a sure thing, but you can call with the computer's serial number at hand (required) and ask if they can furnish system install/restore disks. The charge used to be about US$16 per disk.

Here are Apple's "bootable installer" instructions:

How to create a bootable installer for macOS - Apple Support