Where can you purchase a CO meter to set the idle and load on a 13 HP Briggs and Stratton motor?

Hi, Engine model not specified, but I gather it to be a 28 cu. in. engine. Idle is done with a tachometer and should be 1800 RPM +/- 50. The mixture is generally only adjustable on the low speed circuit thanks to the EPA. Idle speed adjusted above the venturi with a Phillips/slotted screwdriver. Low speed mixture screw is near the intake manifold, usually at a 45 degree angle and having a limiter cap (unless removed.) Adjust idle after engine has warmed five minutes at full throttle. Then adjust mixture for maximum speed, if available(may seem to have little effect or no effect if limiter installed. Accellerate to full speed and use hand on linkage to manually bring to idle. Engine should accellerate smoothly when released if correct. No CO meter required. Peace, crigby