Ha! I've done the same thing. What was you first indication that the bolt had come off (other than the unexplained "thump" you heard 15 miles back)? I bet it was that your "charge" light went on. They should really have used some sort of positive way to lock this bolt from loosening.
Anyhow, If our hosts don't have it, Rovers Down South has them for $7 each. IIRC, there should also be a washer between the bolt and the pulley.
ETC7934
--Mark
1973 SIII 109 RHD 2.5NA Diesel
0-54mph in just under 11.5 minutes
(9.7 minutes now that she's a 3-door).