There are hundreds of options and it really depends on if you need n-1/n-2, or have a preference between Dell, HP, IBM, etc. If you can buy on eBay or Newegg, those are good places to find a variety of vendors for the server of your choice.
If you’re looking for newer servers with relatively higher white-glove service, I’d suggest looking at vendors like Dell PowerEdge Servers, HP Proliant Servers, Xeon Processors and New, Used & Refurbished Servers | ServerMonkey.com.
If you know what you’re looking for and want better hardware prices with little software support, I’d go with a store like The Server Store / used Servers, Workstations & Computer Hardware / Servers, Memory New & Refurbished. These places have good reputations as they get most of their hardware directly from OEM’s or via established asset recovery companies.
I’d be skeptical of refurbished hard drives as you never know what condition those are in, even with a trusted vendor. I’d also be skeptical of refurbished outlets that sell new hard drives, because usually they just buy drives as needed with a large markup. I’d try to get drives from a distributor directly if possible. Newegg also has decent prices on new server drives (at least, they have less of a markup than other stores).
