Princess Auto might be of help. If they don't stock them they might be
able to order them.
If this bolt is accessible and you have A wire feed welder you can try
to remove it by taking A nut and clamping it over what is left of the
broken bolt. Then fill the center of the nut with weld until you have,in
effect, welded A new head on the bolt. You may have to do this trick A
number of times before the bolt will back all the way out. The heat of
the welding process will help loosen it also. I have found A good
soaking with Kroill(this stuff is not sold in stores. I get mine
on-line) If Kroill is not available PB Blaster will help break down the
Air impact wrenches might break it loose better than trying hand tools.
Air seems to apply better circular pressure to get stuck fasteners
moving than the more linear pressure of the human hand.