You don't actually need a caster/ camber gauge. You measure camber, and not caster. Caster is computed, either manually with simple math, or within the tool by taking two camber readings with the wheels turned.
In other words if you go downtown and buy a decent tilt/ angle indicator, you can easily set that up to measure camber
The "usual" way to measure/ calculate caster is to turn the wheels in from straight 20*, take a reading, turn them out 20* and take a reading. Subtract the readings and multiply that X 1.5.