all the Sharp remotes are *similar* in the way they work, basically being a resistor network, but Sharp changed the resistor values from the 860 to the 1000 series, and may have done so before, but the good news is that if you search the forum you should be able to get the wiring diagram and thus be able to either make your own or modify a remote from a different generation.
if memory serves correct one of the creative labs or similar could also work
if you want to get really exotic, a USB keypad, and IRDA or even a bluetooth keyboard could probably be made to work.