These are good. $13 and free shipping. http://www.amazon.com/gp/product/B00...?ie=UTF8&psc=1
WCI Quality Bicycle Computer/Atech
re: Walmart bike computer fritz. The battery could be about dead, have corrosion or a bad connection. Take it out, maybe clean the contacts and put tiny drop of dielectric grease on there. Dollar Tree and other dollar stores often have 3 packs and 1 packs of the batteries for $1. Write your tire size number on the computer so you don't forget it.
Make sure your magnet is lined up with the arrow on the sensor, and close enough to reliably cycle it, or the computer will go nuts and make weird displays.