You could probably buy a NAS/ethernet connected hard disk housing and the hard disks separately and then assemble your own. My first port of call for checking prices would be eBay but other online vendors such as elara, Marx Computers, Komplett, Pixmania, Expansys etc. might also have something...
Komplett currently have a [broken link removed] for €110 (this weeks special). Couple it with a [broken link removed] for €95. All excl. delivery mind you. In the past when looking for something similar, I was going to go for something in the Buffalo Linkstation range. Elara have a 250GB drive for €206 (again, excl. delivery). But the NSLU allows you to add another external USB HD down the road (2xUSB ports).