Do you want a DVD drive to burn DVDs or just to view and run DVDs?
If you want to burn and view you can buy a all-in-one unit with case and drive or you can buy a separate case and drive and assemble it yourself.
Which would you prefer to do? The all-in-one units usually a USB powered. The ones you assemble use either USB power or separate power supply.
Here's a nice one from Asus which is at Newegg:
ASUS 8X DVD-ROM 24X CD-ROM USB 2.0 Slim External DVD-ROM Model SDR-08B1-U
It's $36.99 plus $1.99 shipping and has a $10 mail-in rebate.
To the best of my knowledge, Western Digital doesn't make external DVD drives.