The LaCie Little Big Disk Thunderbolt Series has worked well for me (
http://www.lacie.com/us/products/product.htm?id=10549). It's one of the cheapest Thunderbolt RAID units you can buy. The only problems with this drive are: you may need to purchase a $50 Thunderbolt cable because some models don't come with one in the box, it requires a power adapter (it's not very portable), and it has a very loud fan (which I actually removed).
Another good option is the LaCie Rugged USB 3.0 Thunderbolt Series (
http://www.lacie.com/us/products/product.htm?id=10599). It's bus powered, it comes with a USB 3.0 cable and a Thunderbolt cable, and it doesn't have a noisy fan. This is probably the most utilitarian Thunderbolt drive you can get, but it's not as fast as a RAID unit like the Little Big Disk.