I typically shop the weekend circulars.
Always a loss leader available.
Or Newegg.com
320's for 69 is a good price.
What is your NAS built from? Are you using a PC in a standard case or a dedicated device?
Interesting product.
InstaRAID IR12TB - Internal RAID 0/RAID 1 2 bay 2.5" Hard Drive Enclosure
Another,
Portable Dual Drive RAID enclosure from Addonics
If Firewire were available for connectivity I'd opt for that.