My preferred vendor is Silicon Mechanics: http://www.siliconmechanics.com/
They sell SuperMicro equipment, they list prices, estimated power draw, etc, all up-front. You can add machines to your shopping cart, checkout, and you're done.
They've always provided us with fantastic customer service, and stepped up to replace/repair anything that didn't work right, even if the cause wasn't entirely clear (such as a hard drive issue that could have been OS related, but turned out to be a Seagate firmware issue).