Seriously...most households have a small cake pan. If you don't have a custard cup, then a mug of any sort will do. ( C'mon, be resourceful now :) ) The only thing that is getting dirty is the mug with the egg in it. Boiling water can be done in the microwave in 3 minutes.
And, truth be told, you can poach an egg in the microwave, but I really think that is cheating :-D
FWIW,
Peace,
Amy