We build our own servers from mostly Supermicro components. We usually order from superbiiz.com as they have good prices and are close enough (northern CA) that their standard shipping will get us parts next day.
We usually shop around for hard drives to find the lowest price. Superbiiz has been among the lowest cost vendors the past couple of times I've shopped around.
Occasionally we will check eBay for equipment that is one generation old.