As someone who had injections before and then tried oral, I can confirm what MrsD. says. It's cheaper and less hassle.

If you continue to inject, however, B12 injections are preferably done intramuscular. Best place (as it's the largest muscle area) is the lower back just above the buttocks, but since that is impossible for self injection, the second best choice is the thigh.

Of course, this depends on the type of needle you are using: if you are using the short ones (I forgot the technical details) it won't really matter; the needles for IM are a bit longer, but can also potentially damage nerves if you miss the correct spot.

I have never ever heard about injecting in the stomach, even in long discussions with medical personnel about the best spot for B12.