Depends on your budget and system requirements. If you're using onboard video, you might get away with a low end unit at around 400 watts. Here's a good value and quality unit from newegg: http://www.corsair.com/builder-series-cx430-v2-80plus-certified-power-supply.html
($25 after rebate at newegg). You can use almost any standard atx ps; only a few are specialized for smaller cases. The corsair is a good brand. Others are antec, seasonic, ocz, enermax, and xfx. Your vender should have one of these brands.