Try one of these: http://www.directron.com/matxps.html
Base the output wattage on the (reliable) result from the above mentioned jscustompcs website.
If you can't find a powerful enough small PSU, consider buying a larger PC-case as well.
I think this
is about the best matx psu you can get. Micro atx motherboards(I assume you have a matx on if you need a matx psu) are designed to need less power, so a relatively weak psu can do the job in these types of systems. But I would definitely recommend that psu, first, it's pretty powerful for matx, second it's good brand, Enermax.