I had the same thing happen to my 91 civic. What I did was to take the mount itself off by unbolting the mount from the engine (two bolts I believe). Using heat and a BFH, I was finally able to get the bolt out. Then I just went to the hardware store and got another long bolt and nut.

 

Yeah.. the through bolt on the bottom. I took a dremmel tool and cut across the nut and then drove what was left of the bolt back through with a screw driver.

I ordered the replacement through the Honda place near my house. The Honda part has an odd end to the long bolt that keeps it from spinning when you tighten down the nut.

Shown on this attachment as "Alternator Through Bolt"

 

I used a socket on each end of the bolt and nut to tighten it. I ran it for about 20,000 miles with no problem. The nut and bolt cost me about a buck.