The name of the part is a cable stop or housing stop. Usually they are brazed or welded right on the frame instead of being an add on block like yours. This means that it's not a regular off the shelf part at the bike store so if you shop around you may or may not get lucky in finding a replacement. If you're at all handy with home shop tools I'd suggest that you just make a new one from a small hunk of aluminium or even steel. I wouldn't trust to make it from plastic as the same thing may well happen again depending on what sort of plastic you have or get. The 15 go 20 miniutes it would take to make something identical in metal would be a lot less time than driving or riding around to all the bike stores looking for a replacement.