FYI,
It's not often that Canadians have manufactured goods that our cousins
to the south don't. In Canada, you can buy 48" fitted bed sheets (and
mattress covers) i.e. the same width as the upper bunk bed of your
Vanagon Westfalia. They are called "3/4 size" and they measure 48" by
75".  They are meant for a normal bed mattress of about 6-7" deep so
they fit a bit loosely. The hinge in the upper bed does interfere with
the fitting but not much. The "3/4 size" is not as common as regular
single/twin/etc. but they are available. We bought our sheets from Sears
Canada, catalogue no 962 051 609 C at $C 27.99 or about $US 19.50. You
can probably get your local Sears to order them for you and save on S&H
charges or you can try your luck with a direct order at 1-800-267-3277.

No, we don't own any Sears stock; we've never worked there; we don't
know anybody who does.
Larry and Avery Johnson