It's really nothing more than a LARGE allen wrench...perhaps you can find one at Lowes, Home Depot, etc...
If the plug is plastic, and since it's been installed for a long time, probably with some thread compound to keep it from leaking (yah, right!)...be careful as it's really easy to 'round' off the socket of the plug...
If the flats get rounded off in the plug, you'll have to carefully drill out the old plug without damaging the threads in the tank...OR, try this;
I've sometimes used a flat bar that will fit tight across the hexagon, and use an adjustable wrench to unscrew the plug...this is probably the best method - then get a new plug, and use lots of teflon tape to seal the threads...
Ray & Pat; Morada, CA