Unfortunately Apple only sells the sleds with hard drives already installed.
Example The prices are fairly high but they are 'server' class drives and have a warranty. You can hunt down used sleds or sleds+drives on ebay or via used hardware sellers. If you buy empty ones you can put in your own drives, but don't expect any support from Apple. With a little hunting around you can snag empty used sleds for around $150. I snagged 2 used ATA sleds for my old G4 xserve about year and a half ago for $120 each off of ebay. I threw in 2 Seagate 500GB drives and they have worked fine for me since. Hope this helped.
