Try to find a MicroATX PSU... if the wattage is a bit more, don't worry about it. Just make sure (I ran into this a couple weeks ago) that the leads that go to the mobo are on the same side as the one you have.
There are 2 different MicroATX supplies, and the wrong one will be a pain in the arse to put in.
Go to a local comp store (may cost a bit more) and take the PSU with you. (NOTE: CompUSA or Best Buy don't count.)