@Delightful if I were you I probably wouldn't give a damnation if I'm allowed to call the plumber. The situation is sirious. You wouldn't wait with calling a specialist if your gas oven was leaking gas, would you?
Plungers can usually be bought in stores like Castorama, where tools and materials for building are. But maybe ask Google or your neighbors, that would be easiest I think. Hardware stores have them too.