Yeah, they are pretty common - I actually got one two weeks ago... I am in Oz, so I can't give you US links, but you will get an idea of what to look for. Usually they are under "External Harddrives" or "Enclosures". If your old HDD is from a desktop (not a laptop) you would want a 3.5" enclosure.
The options you need to think about are how it will be transfered - by FireWire or by USB2.0...FireWire (of Course) is more expensive...
Try these ones:http://www.eyo.com.au/items_CatID_3_SubCat..._Enclosure.html
...and these ones...http://www.computeralliance.com.au/parts.aspx?qrySubCat=HBK
...and this one (the ones labelled 'enclosure')...http://www.i-tech.com.au/products/10_Hard_Disk__Storage.asp
..and there is some in here...http://www.megapc.com.au/index.php?ct=Remo...subcat&ix=8
...and more still.....http://www.pccasegear.com/category287_1.htm
...lol, this is funny....http://www.pluscorp.com.au/Content.asp?CategoryID=317
..and, surprisingly, some more...http://www.techbuy.com.au/searchcat/HDD_EX...DRIVE_CASES.asp
...I have the Black Nexstar 3 USB 2.0 3.5" enclosure. It is a snug fit for my old HDD, but works a charm. The only thing to remember is that very few of them are bus powered and require being plugged into the wall...
Hope that helps