We've recently researched replacing or expanding drive modules for Apple's Xserve RAID. The results are not encouraging. The only place we've found modules is on eBay.
We did find a forum post in August 2009 that stated "Apple store doesn't sell them anymore but if you phone AppleCare they will sell you one as long as you send the failed drive module back." However, this company has taken a very clear stance: Hardware Advisory - Replace your Xserve RAID's Immediately
The rumor mill is that Apple's drive manufacturer for this product has disappeared and even Apple can't get these drive modules.
If you have an Apple XRAID but you're not ready to retire the device, then we encourage you to purchase a few extra drive modules. Beware the $20 Blank drive modules. These are for cosmetic purpose only. They are used to fill the void in an Xserve or RAID and do NOT have hard drive connectors. All Apple Xserve RAIDs require Ultra ATA (Parallel ATA) Drive Modules in the following sizes:
• 250 GB (PATA)
• 400 GB (PATA)
• 500 GB (PATA)
• 750 GB (PATA)
Contact us if you have any questions.