Where can I find a Key Cover and Key for this Danish Modern Cabinet?  

My guess is that you will need to replace the whole lock set. Or perhaps you will get lucky and find a new lock set that has a keyhole liner that fits. Although it just occurred to me that I don't know that lock set come with the key hole liner. Something to research anyway look up something like "mortise lock skeleton key"
As for the key, well, a replacement can be cut by a locksmith if you can find the keyhole liner.
As a final thought maybe someone could make you a keyhole liner out of a hard contrasting wood like ebony. Or perhaps a machinist could cut one in brass.

Woodcraft has keyhole liners, but either the brass or the recess, or both, would likely need to be modified to fit snugly.
Edit: Lee Valley offers them in three sizes, too.
http://www.leevalley.com/en/hardware/page.aspx?p=40633&cat=3,43496,43491

Thank you for your posts. I've had this bar cabinet for 30 years now. It was a thrift store find for 25.00......I think it is time I get it fixed

It is also a very nice piece of Danish Modern, so be careful what you do to it. Designed by Tove and Edvard Kindt-Larsen for Gustav Bertelsen.