You can use nearly any flash with center pin. Just make sure its trigger voltage is below 25V. Many old flashes have very high trigger voltage (even higher than 100V!) that could damage your camera. The problem is that all these flashes are non-TTL. I'm afraid, there is no 3rd party flash with TTL support. Since the GRDIII, Ricoh uses their own TTL flash protocol incompatible with anything on the market. So all we can hope is a decent Ricoh flash powered by just 2AA batteries and tillable head. The existing GF-1 is too big and heavy even for GXR.
The flash Ricih GF1 is a bit bulky, but not much more than competing flash power.Toutefois even that's not really a problem in my opinion because the GF1 allows very efficient mode: you place the flash some hand in the room where you are, so that the capture infrared flash sees the light of small flash incorporé.Il retain TTL operation, will give an indirect lighting abiance high quality, and will sound exactly as if he was on the device. you can dose the proportion of both flash at your convenience: you a lightweight and mobile in their hands: what else?