I would recommend going and talking to an ortho about this.
oops,,,i'm sorry.
you want supplements. i misread you.
what kind of forearm exercises are you doing?
reverse curls or any kind of flex that works the flexor and supinator?
i can't think of a whole lot of workarounds. howz about those hand grips,,,or balling a newspaper into a small ball in one hand,,,or holding the end of a broom and flipping it from side to side with your forearm parallel to the ground.
maybe isometrics?