The only people who need to be using a vitamin B12 injections are vegans. Vitamin B12 is the only vitamin/mineral that is only in animal food sources. There are no vegan sources of vitamin B12 available. Vitamin b12 deficiency is very common in vegans as a result.
The best sources of vitamin B12 are whole eggs and red meat. It is also in poultry fish and dairy products, just in small quantities.
If you are a vegan, you are going to need to use a vitamin B12 supplement. If you aren't a vegan, include lean red meat (ideally grass fed) into your diet 2-3 times per week. I would also eat 2-3 whole eggs every day. Don't be scared about the cholesterol in eggs. Dietary cholesterol actually have very little to do with raising cholesterol levels in the body. Poor cholesterol levels are due to a diet high in sugar, refined carbohydrates and trans fats. Whole eggs are actually very healthy to eat!
Any bodybuilder who is regularly eating animal protein sources will be getting way more then enough vitamin B12 in their daily diet. Using extra vitamin B12 can actually lead to toxicity symptoms.
Rather then use injectable vitamin b12 (which you really don't need), I would get a good multivitamin/mineral supplement. That way you ensure all your daily vitamin and mineral needs are met, not just vitamin b12.
Check out ruthless 3d from n2bm.com. That's the multivitamin I use. Really good product and very affordable.