I am not following methane sensors technology, but in general low concentration almost always means one needs high sensitivity, high sensitivity almost always means costly sensor. With atmospheric methane levels around single ppm you are definitely in the very low concentration area.
There are specific cases where this is not true, but with methane being rather simple, highly symmetric molecule, I am afraid you can be out of luck.
You might want to try to contact university in your area, preferably look for analytical/spectroscopy department.