There are several ways you might rescue this. For all, suggest using lots of PB-Blaster or Kroil to chemically allow some loosening. Also, some heat on the carrier may be helpful. Note that, not being immediately familiar with your exact solution, my suggestions are somewhat generic and apply to any bolt that has broken off inside something.

First, drill it and use an easy-out. Be very careful about the easy-out, they are extremely hard but also very brittle. You have to have a straight shot at the easy-out, or it may bust off in the bolt and they yer screwed 2x.

Second, it's a through hole, drill the bolt and the heat and vibration may be enough to loosen the grip and it will spin right through with the drill bit.

Third, and if the hole isn't through, many of us nowadays have reversing drills. If not, a cheapo at WalMart may be enough... _IF_ you can find a reverse drill bit. The bit should be a little less than the diameter of the bolt so you don't cut the threads, but often if you can find such a thing, the heat and vibration of drilling will be sufficient to back out the busted bolt.

Once in a while you can weld "something" to the bolt that will be sufficient to turn it out.

There is also, rarely, a machine shop service that uses some kind of high voltage magic to remove bolts frozen in. You'd have to contact a machine shop for this to see if they have it, and take the part to them. The likely means disabling your car for a while, but you're at that place already.

As for another bolt, the dealer is the place to go for mission-critical parts. You may be able to read the hash marks on the bolt head and determine the grade of the bolt and get it at a hardware store, but BE SURE that you get at least the same grade. DO NOT substitute any common galvanized or coated bolt simply because "it fits". DO NOT use anti-sieze upon reinstallation unless MBz does it that way.