Hi there
I offer it as a mobile therapy. It's quite tricky to get the timings right, etc, but here's how I do it.
If I'm out, I set a timer for the heating unit to come on (just one of those you plug into the socket) to heat the stones, then, once they have reached temperature I take the stones out, put a filled hot water bottle into the bottom of a 'keep food cold' bags for picnics, wrap the stones in a towel, put another hot water bottle on top, with another towel. I then empty the rest of the water away, pack the stuff in the car, including the heater unit (and an extension cable in case their plug is a long way from where you are doing the massage and a folding table, to put the unit on).
As soon as I arrive at the client's, I plug the heater in, empty the hot water bottles into the unit, add the stones, ask for a kettle full of water (my unit is huge) then get the rest of my stuff out of the car, do the consultation, and you should find that the temperate is about right.
Lot of clat, but if you charge enough, it's worth the hassle, and it's quite popular with my clients.
Hope this helps, and there may be a simpler way, so feel free to let me know of a more user-friendly manner to do it.
Lesley