They are available from Buckeye performance and are rated about 55 amps. The stock ones are rated 30 amps, IIRC. http://www.buckeyeperformance.com/p24.htm
Some dealers may do the job, but I would either do it myself or take it to an independent shop to get it done. I haven't done it but I'm guessing it is probably a 4 hr job @ whatever the going shop rate is in your area.
skydoc_17 on this forum also sells an upgrade kit that includes the stator, gaskets, upgraded regulator/rectifier, and needed wiring.