You can take quite a lot of b12 before having any problems because it is a water soluble vitamin. What your body doesn't use it flushes out. If you going to inject every day with your gear, you may want to keep it around 500 mcg /day. That'll give you the appetite to eat more without overdoing it on the b12.
You can get b12 at some vet supply places online for a pretty cheap price, but I got mine at synthetek.com. Their product has a higher dosage, 2000 mcg /ml so you'd only have to inject .25 cc to get 500 mcg. It is higher priced, but it works quite well, and I'm satisfied. A full injection of 2000 mcg only has to be done every 3-4 days.