Unless the quantity is large, you'll want to start with a stock box and
have a local shop cut it with a laser or water jet. Many are designed
to take a separate front panel insert, which you can source pre-cut from
a shop like Front Panel Express.
The trade rags like EE Times have ads for enclosure makers; some popular
ones are Bud and OKW, and the usual distributors offer many (but not
all) of their lines - see Digi-Key, Newark, Allied, etc. for a starter.
At 2-3 rack units and above (1.75" per RU) there are a lot of options
out there - look for "enclosure". At the 1RU size, the options are a
lot more limited for some reason. There are shops that will make cases
for you custom, but at ~$200 apiece for a short run; that's a lot of
money for something that should be a stock item. At this rate, it's
easy for the enclosure to be 50% of your bill of materials.
Something else to consider... buying a pre-fab unit with power supply
and fan may be a ton easier/faster/cheaper than rolling your own
solution. (It's not hard for the mechanical aspects to take as much
effort as the electronics.) Also, don't forget about safety (e.g., UL)
and FCC certs.
Here's a collection of links I've compiled over time...
http://www.okwenclosures.com/catalogue/metal/info_pages/mettec19info.htm
http://www.sescom.com/enclosures/rack_chassis.html
http://www.altronics.com.au/index.asp?area=prod&grp=220
(I like the H5035, a steel box with a nice beveled face.)
http://www.chassis-plans.com/custom/custom-showcase.html
http://www1.jaycar.com.au/productView.asp?ID=HB5120&CATID=5
http://www.lansing-enclosures.com/
http://www.web-tronics.com/1usteelchasw.html
http://www.jdr.com/interact/item.asp?itemno=GR-3120A
http://www.protocase.com/products/standard.asp
http://www.industrial-enclosures.com/html/19-inch-enclosures.html
Front Panel work:
http://www.frontpanelexpress.com/home/index.htm
http://www.polycraftinc.com/samples.htm
Server cases:
http://www.rackmountplus.com/list.asp?MENU_SEQ=10101&PAGE=3
http://www.servercase.com/
Good luck!
Richard