IDE is a standard so it's going to have 40 holes, but some device manufacturers key their connectors with 39 holes so you can't put the cable on upside down.
All it is is a regular ribbon cable you use for a desktop computer's harddrive and cd/dvd drives.
Like MPCHunter said... some have 39 holes and some have 40 holes. If you get a 39 hole cable, just use a sharp pin and push it through to make the extra hole.