Mouser Electronics sells a variety of fans, and has decent data about
the current draw, CFM rating, dBA (noise) rating, etc.  There is a
looker-upper at
http://www.mouser.com/search/Refine.aspx?Ne=1634435+1447464&N=1323038%201634435&Ns=P_SField&RefType=Header
, or find one fan in the looker-upper and click on the "catalog page"
link to see a PDF of the actual catalog page, which might be a little
easier to use for comparison.

Digi-Key also sells fans, but they have a surcharge on small orders
which Mouser doesn't have.

Fans are also a common item at surplus electronics places.  You might
get less data, but if they give you at least the manufacturer and model
number, you might be able to look up the specs at the manufacturer's
site.  The surplus places are generally a lot cheaper, as well:
http://www.allelectronics.com/cgi-bin/category/220100/D.C._Fans.html

Just about any desktop computer will have at least one suitable fan in
it and maybe two.  These will most likely be 12 VDC or possibly 5 VDC.
If you can get an old junk computer for free or cheap money, it may be
worth it - you get a cheap fan or fans to experiment with, and if the
hard drive is still in it, that can be torn down to yield a really
strong magnet that can be handy for removably mounting stuff to your
van.

If any of the fans you run across have three wires, the third wire is
probably for a tachometer or temperature sensor.  Computer motherboards
use this to determine if the fan is actually turning and/or the air
temperature coming through the fan.  For Vanagon purposes, you can
ignore the third wire - don't connect it to anything.  There are various
color codes but often the black wire is ground, the red wire is +12 V,
and the other wire is the tach or temp wire.  If you don't know the
correct hookup, you are unlikely to hurt anything by using a 9 V battery
and trying different combinations of wires until you find the right one.
The fan won't run at full speed on 9 V, but even large fans should at
least try to start turning when you get the right combination of wires.
Once you know the right hookup, you can try it on a beefier 12 V supply.

Matt Roberds