Best legals is Vitamin B5, B6, B12. You can also try natural foods that boost your endurance like beet root juice etc.
Illegals Equipoise, EPO. etc..
Best off trying out something like an altitude tent though if your a boxer there's certain gyms now days that have special oxygen enviornment like the altitude tents do. Don't know much about them or where you can find a gym like that, but alot of athletes (boxers in particular) do use them alot for training.