RBRacing.com has a decent one but ensure you have the four wire heated o-sensor. Most guages are not worth much. Needs to be LED with 7-9 LED bars green yellow red. Need to visibly calibrate against a real dyno with AF so you know what the reading means for your bike before adjusting fuel. Lambdaboy.com has the only and most accurate wide range meter and interfaces to laptop but looks like hell on a street bike.
Any O2 sensor would be helpfull. Heated is a bonus. Just remember that they are not very accurate, unless it is a "Wide band" ($$$). Try to stay as absolutely close to 14.6/7 as possible, error on the side of rich.
If it is not heated, you will need to get it hot first with a good ride.
www.diy-efi.org has some instructions on making a reader for an O2 sensor. It could also be done with a voltmeter.