You would be able to get B12 shots from your physician. I'm not sure which healthcare plan you have, but I have Kaiser Permanente and they supply them. Keep in mind this is approved via your MD, not just because you want them. ;-)
Why do you need B12 shots? Most doctors won't just "give" them to you unless there's a need.
I would think that any doctor's office stocks B-12 shots. My Dad was a physician, and he kept the B-12 ampules and syringes in a sterilized tray in the refrigerator. If you locate a physician you wish to visit, inquire on the phone that B-12 shots are available.
Harrald