I'm probably wrong here, but I think the P4 "M" chips are mobile
processors, so you'd probably have to buy a notebook computer to get
one.
You can probably find one by looking whereever notebook computers are
sold.
If your plan is to pop one into a desktop system, or to replace a dead
chip in a notebook, your best bet would be whoever made the notebook.
The mobile chips aren't generally sold bare to end users.
