A quick search turns up a picture for a 475W PSU (Part no F217J, VP-9500073-000) on the river site which seems to indicate that it's not a standard ATX PSU.
Another site sells a 500W PSU from Enlight (Part no J860K DPS-360FB-1A PS-5361-2 PC) which they say will do the trick, and that has the dimensions of a standard ATX PSU.