Woot offers BLACK+DECKER SF100 Wood Stud Finder for $5.99 free shipping with Amazon Prime. 

Product Description: 
Simple yet effective, the Black and Decker Stud and Metal Sensor is a must-have for any tool box. This easy-to-use device is incredibly accurate, despite its size, and can locate studs up to .75 in. thick. LED lights on the stud finder light up to accurately indicate the location of studs.