Which brace should I use to lessen the pain of osgood schlatter in my knees during sports?

Knee strap. There are not any good studies that support brace usage however a knee strap may be helpful.
Knee pad. Unfortunately, there is no specific brace for osgood-schlatter's disease. The pain is worsened with direct contact to the swollen front, top of the shin bone. Hence, padding this area is your best bet for reducing pain. Quad stretching and ice after activities reduces the pain from the tendon tightness, which is not addressed by bracing. Ibuprofen reduces the pain and swelling.
You can't have OS. If you are 21, your growth plates are closed and the diagnosis of Osgood-Schlatter's is not possible. More likely you had OS as an adolescent and now have a painful ossicle. There is no brace. You probably need surgery to fix this.
Cho pat. A brace like the cho pat that goes across the mid and distal (end) of the patellar tendon can both unload (take tension off) and cushion the tendon and its attachment.