Computer power supply and psp?

my computer power Supply is on the bottom and the ATX power connectors on the top. what power supply should i get . i want a cheap one .I have so far is Motherboard http://www.bestbuy.com/site/Asus+-+Micro+ATX+Motherboard+1200/1066/800/667MHz+%28Socket+AM2/AM2%2B/AM3%29/9724401.p?id=1218159864407&skuId=9724401 and case http://www.walmart.com/ip/Antec-Three-Hundred-Case-Black-finish/10104855 and i have a phenom cpu. and my psp pandora battery i stick in psp and sreen black and green power light on.what do i do.