You can get an enclosure at your local consumer electronics store or on the interwebs at sites like newegg.com or tigerdirect.com etc. Just make sure the enclosure is for the type of hard disk you have and has the connections you need.
if you want to be able to use other drives with your mac. I have been using one because I have several older drives. It's nice because it will do SATA and IDE, and will handle laptop and desktop sized drives.