here are some guys I've used
René Guardiola, YourManInPanama.com
has new, clean, air conditioned 12 passenger bus, speaks excellent English, is a licensed tour guide. Has a couple of other guys/gals with the same 12 passenger vehicle.
Pancho, panama cellphone #6-681-6873
has an older Galloper that seats 4 or 5. Speaks good, but accented English.
Has a steady business driving gringos around Panama.
Might be able to arrange a larger vehicle.