Can I replace the upper alternator bracket and if so, where can I find one?

you need to stop driving your vehicle right away and get a "taps and dies kit", which will help you get the rest of that bolt out so you can replace it with a new one, brackets can be found at most salvage yards and purchasing a repair manual for your vehicle by year make and model would be a plus. Automotive repair shops can do this for you at a price, but if you got any skills in the area of using a drill, possibly impact type for heavy duty jobs, then getting the best possible angle is the key cause you dont want to ruin the original hole, with the kit, you'll be able to tap into the remaining piece of the bolt/screw by positioning the bit on the impact drill, which should have a handle on it's side for extra stability while your working, right in the center of the bolt/screw, pressing firmly and starting of slowly, to a moderate speed will get you in there, be sure to get in as level with the bolt as possible. continuing to drive the vehicle with that extra tension on that one bolt alone is the road to disaster and could damage much more if it were to snap while driving especially at a high rate of speed. get it done quick. good day.