Most of those portable roadside displays are powered either by solar PV panels charging a battery, or (the larger ones) by a small diesel generator/battery setup. I haven't heard of a fuel cell being used for that application yet.

There are 2 types of fuel cells in common use, hydrogen/oxygen and methanol/air. the H2/O2 type would need a source of compressed gas, but the MeOH/air type runs on liquid fuel and oxygen from the ambient air.