You probably have an ATX PSU. ATX dimensions, based on the website Wildcat gave you are, 5.9" x 3.4" x 5.5". Not exactly what you have (5.75 x 6x 3.5 but pretty close. So any ATX should work.
I could find any ATX 375 watts so here is a list of 400W ATX PSUs, Newegg.com - Computer Hardware, Power Supplies, ATX 400W
Hope this helps and good luck!