There are complete drives available for sale on eBay that are shipped with the appropriate caddy so all you have to do is stick it in your 600/E/X.
There are also bare drives sold on eBay specifically for the 600/E/X series. The sellers usually offer to send instructions on how to pull your current drive out of its caddy and replace it with the new bare drive.
You can also try to find a bare drive on your own. For more information about this, you might want to consult the following thread, especially if you currently have a Toshiba-branded DVD drive and you would like to continue to use your current "front bezel":
Find the right optical drive to fit 600X