Most MOTU products use an internal switchable power supply which accept a standard power cable. However some older units did have "wall wart" power supplies which are not as simple to replace. Here is a list in case yours succumbs to entropy:

MIDI Timepiece (original):

• 9 Volt, 500 mA, Positive Tip

Pocket Express:

• 9 Volt, 300 mA, Positive Tip

MIDI Mixer 7s:

• 18 Volt, 500 mA, Positive Tip

In addition, one of MOTU's newer firewire audio interfaces, the Traveler, has the option of using a "wall wart" DC power adapter with the following flexible specifications:
• 10-24 Volt, Positive OR Negative Tip