Those plastic housings and spring assembly are $3.00 each at our shop however, I have been told that once ours are gone they are gone. Supposedly they are no longer availible from Shimano. If you do go to a LBS bring the old assembly with you as there were several different varietys.
IMHO... For the price of a new set of canti brakes just get a new set of brakes. You get new pads, new springs and you won't go nuts looking for a part.