I never located the part, and ended up paying for the Apple adapter. You might be able to get by with an Apple adapter, as they have 50-pin IDE connectors, like the HP.
This site has the two different iMac adapter boards.
Scroll down until you see:
CD / DVD Adapter Board - $25.00 - 922-4027
iMac (Slot-load CD/DVD / Summer 2000)
CD Adapter Board - $15.00 - J1066
iMac (Tray-load)
If you're adapting a tray-load drive, the J1066 should work.
Curious, what do you need this for? Some sort of HP slimline?