Just my advice on the subject. I think supports just do a little help, which is good and nice, but don't rely on them. Problem should be treated through better body knowledge, body care and good guitar technique, working with a good guitar teacher and a physiotherapist. Be careful with some accessories that force some positioning or simply release unnecessary pain, since they will encourage you to keep doing things wrong or to do them even worse. Feeling pain is actually a good advice from the body, don't simply release pain from external medicine or device, try to solve internal body root cause.
With that said, I actually like the arm sleeve with the piece of plastic below it, when using traditional posture. Besides comfort from guitar edge, I think it makes arm sliping easier for active hand positioning and also seems to get more sound from some guitars (preventing direct arm-flesh contact will let guitar vibrate more).
There is a band you can put on the forearm over muscles to release their tension, or an accessory you put over wrist to keep it "right". I don't recommend such things. You might use them eventually, trying to feel what happens and trying to learn from the feeling and checking your body. But don't use them systematically, don't let them build uncontrolled and unconscious reflexes.