I have a 3D printer that is going crazy with x-axis shift, and I need a tension gauge belt to measure the tension.
I've never used one before, and looking online, I can't tell which one would be the right fit.
Any ideas? What things should I look for?
It's extremely unlikely that belt tension is actually your problem. I've never heard of anyone using a gauge to measure their belt tension. Typically you just pull your belt tight by hand so that it produces a low note when plucked. It's far more likely that you're experiencing shifts due to too high or too low stepper current.
Unless your belt is so loose that it easily skips over the pulley (which should be obvious without using a gauge) or so tight that it completely binds up (it would be impossible to get it that tight without some kind of superhuman force) it's definitely not the problem.