Handheld response keys


We are looking at completing some cognitive tasks during exercise (running on a treadmill). Specifically, we are looking at using the Eriksen Flanker task/Stroop tests and as this is done during exercise, the response pads aren’t suitable for this. We need two response keys (left and right hand) and it would be beneficial if these were handheld.

Does anyone have ideas regarding purchasing/developing any handheld response keys as opposed to using the common response pad setup?