Originally the bus had one key that fit the ignition and all the doors.  If the ignition key doens't work the doors, something has been changed.  The ignition and driver's door are the most common locks to go bad and need changing.

Anyway, I've got a VW key clipper and code book.  If you can get me the key code, I can make you one.  Someone stole most of my key blanks a few months ago, but I still have a few for late buses.

IF no one has changed any of the door locks, the easiest one to get the code from is the sliding door.  Take the inside handle off and pull the outside handle out.  In the middle somewhere you will see the code stamped into it . . . it will be two digits, a letter (I think it's an M, but it's been a while), followed by three digits . . . something like 24M104.

Go here:

http://www.volkswagen-classic-parts.de/ and click on "English" at the top of the page.  Then click on the "Keys and Locks" section. 

There's a lot of other stuff still available through VW.  Depending on how good your German is they've got the parts catalog online through the German-language site.