Antec Earthwatts 380-Watt Power Supply for desktop PCs for $40 with free shipping. It usually sells for around $50, has a excellent reviews and reliability record. 380 watts is plenty of power components in most basic desktop computers, but you can use their wattage calculator to check.

Also a Rosewill 400-Watt PC Power Supply for $33 with free shipping and great reviews. When buying a power supply, pay more attention to its reliability reviews, and less to maximum wattage rating. Power supplies with better reviews can actually power your computer more reliabily than ones with higher wattage ratings.