I'm renting some co-location space for our web server.  We have a
"tower cage" with just enough width for an upright tower.  However,
it's 30" high and could likely fit 2 microATX cases stacked on top of
each other.

I want to maximize our usage of the co-location space, but most
MicroATX cases come with a 150w power supply and I'm worried this
won't be optimal power for our server hardware.  I want to know where
I can get a case that is under 15" high, fits the standard motherboard
form factors and has a better power supply.

The quality of the case is important too.  More so than price.

In case you did not know the motherboard specs:
MicroATX: 9.6 in x 9.6 in
ATX: 12 in x 9.6 in
Mini ATX: 11.2 in x 8.2 in

Another useful fact is the following conversion: 381mm == 15 in

I would try the following url:
[ http://store.yahoo.com/aaronix/atxcases.html ]

This is a link to the compact case section of Aaronix.com.  Any one of
the cases in that section appears to meet your criteria, and they are
all configurable to include a 300 w power supply.  I have never
purchased anything from them, so don't take this as a personal
recommendation. ;)

You can also try looking the old-fashioned way:
[ http://www.pricewatch.com/ ]

There is a good article on choosing computer cases here:
[ http://www.directron.com/howtochoosca.html ]

good luck.

Even if you find a 300W PS for the microATX, you're oing to have heat
dissapation problems. Part of the reason for building servers in
towers is to allow for air-flow.

morris-ga