For forearm support, hands down the best for me was the old Lou Scalia Bowling Splint. Unfortunately, if you also want/need wrist support it's hard to wear both at the same time, but strictly for forearm support those were the best. I say "were" because the Lou Scalia splints are no longer available; however, tennis elbow braces are basically the same thing. You just need to find the kind with two straps (wrist and forearm area) and a metal bar between them.
