You'll need to re-download it from the App Store (it should automatically be flagged as purchased, though, so you shouldn't need to re-buy it: just check the "Purchases" tab).
It'll download as an application: "Install Mac OS X Mountain Lion.app". If the installer automatically launches, just quit out of it. Right-click on the application, select "Show package contents", then navigate to Contents/SharedSupport. In there, you'll see "InstalESD.dmg": that's the Mountain Lion image.
You should be able to use that image directly in Parallels et al, but if you really wanted physical media, you can burn that image to a disc or create a bootable flash drive using Disk Utility. If you go the flash drive route, you'll need to use the GUID partition table.
