hi im sure last time we were in goa we got one of those little boilers from the elephant shop in candolim. & the time before that we got a couple from the supermarket nr the osborne hotel opposite babutes bar - not the lui ma supermarket but the one across the road.
i know years ago 'pifco' used to make little stick in your cup heaters that you could buy here in the uk but i havent seen them at the airport for ages now - they tend to sell the mini kettles instead. have you tried a google search or maybe even camping type shops???

Why not buy a kettle over there, Dirt cheap, cheap enough to leave there...........