The fusible part of the plug which sticks out into the tank, is an 8mm dia plastic stem about 40mm long, this is attached to a steel bolt that is screwed in from outside, a 13mm 3/8" drive socket and extension is required to get at it in the fins above the burner.
As for availability, I'm still waiting on delivery of new stock albeit it should be any day now?, otherwise, old stock is all but unavailable, as Damian says ebay is probably your best bet, at a price! or no reason why a dealer somewhere might have one?
If though yours has 'blown' but otherwise intact, ie, if the plastic stem is still firmly attached to the metal nut? then I can recharge this for you.
Also to get yourself out of trouble, if your heater is gas or electric heating and as the flusible plug is only for gas heating, then you can block of the hole and use electric only for heating, electric heating having it's own separate over heat protection