Tom Brady, who tore an ACL in his left knee back in 2008, wears a Donjoy Defiance III knee brace in games to protect his left knee. Brady said this about quarterbacks and hits below the knee (via WEEI): “Why I never wore a knee brace before, I have no idea. Why every quarterback doesn’t wear one on their left knee, I have no idea. It’s just so unprotected.”
The Donjoy Defiance III must be purchased and fitted through your doctor, but you can check standard-sized Donjoy knee braces out on Amazon.