Here's an excellent source for hard-to-find power supplies: http://www.atxpowersupplies.com
While they also carry industry standard units, they are especially useful if you need proprietary form factor supplies for certain Dell, HP/Compaq, Shuttle, and other non-standard systems. In addition they have several of the SFX form factor variants available, which are used in many of the smaller pre-assembled OEM systems.
They also carry a number of cases as well as many useful accessories for system builders - including my book, which is nice. <g> Scott.