Should not be hard. But to avoid some confusion, recognize that there are TWO interfaces involved. One is the interface between computer and external case. Choose that according to what conectors you have available. For example, I have a mobo that has a real eSATA controller built in, so I bought an AZIO case that has both eSATA (for my machine) and USB2 interfaces (can hook up to almost any machine).
The second interface is how the hard drive connects into the case. In you situation, you want one that uses an IDE drive, presumably of the 3.5" size. For example on newegg.com, I searched (top left window) for external enclosure 3.5, then in the left-hand column narrowed it down to external enclosures, then to 3.5, then to IDE, and it had 172 of them. The left-hand column offered to narrow it further by External Interface.