Many people wear arm guards, though I've heard said that if you hit your arm, your form is wrong. The Olympic archers wear arm guards, too. Things happen, and if you are hitting consistently, but your shirt gets in the way, then get one of those guards. If you can't find camo, and you want that, then buy some camo tape.
If, however, you aren't consistent enough in your shooting, then I recommend getting out to the range with a person who can watch your form. There are all kinds of reasons for the symptom you describe, and it would be a waste of your time for you to go out and try to change too many things at once. It's probably only one or two small things you can tweak, and knowing them quickly will build your confidence. Again, though, if you are shooting consistently well, and this is your only problem, then get a guard.