This is one of the early Intel powered Mac minis, these used the 110W power supply rather than the 85W power supply that came with the PowerPC based Mac minis. I don't see any of these power supply on iFixit but a quick search found two options through Powerbook Medic and Other World Computing...

Powerbook Medic - $40 110W power adapter

Other World Computing - $46 110W power adapter

For what it's worth the Apple service part number is 661-6386.

Hope that helps!