Apologies for reviving an oldish thread, but this happened to my rear shock mount a couple of days ago too.
I did a temporary fix with a mild steel M14 120mm nut and bolt I had in the garage, and then drove very steadily to the nearest engineering supplies place.
The best bolt they had for the job was an M14 130mm 8.8 strength with matching high tensile nyloc nut, all for the princely sum of £1.75.
I managed to clean up the head of the old bolt and could see it was 10.9 strength, and I mentioned this to the engineering place.
Obviously, his official advice was to replace with the same strength bolt, but that a brand new 8.8 bolt was likely to be stronger than a 12 year old 10.9 bolt anyway.
I got the one with a shoulder that the drop link and shock sit on, and I ground off about 10mm to make it a neater fit.
I've since found a place online that does 10.9 nuts and bolts, so might get a couple as I suspect the other side might do the same one day.
Interestingly, the mild steel bolt had not deformed at all, but clearly wouldn't have lasted long term, I did it only as a fix to get me mobile again to fetch the new bolts, and I don't recommend it!