The grounding probe is usually with the 3 prong, where the 2 flat prongs are plastic and the circular prong is metal. If this is the case, any 3 prong extension cord will do (do not use 3 prong adapters) All it really is, is hooking a wire from your water in the tank, to a grounded outlet, technically you can hook it up to the concrete floor, antenna on the roof, or anything else that is grounded to the earth.
One additional thing, when using a grounding probe, I suggest to use a GFCI outlet. The GFCI will cut the power to your equipment when it detects an electrical leak (which the grounding probe will complete the circuit so the stray voltage can leak out)