If you are an organ player, the EV7. Rather expensive, but it has the same sort of throw length a Hammond pedal does. If you are OK with shorter throw pedals, the EV5 is good.

NEVER use a Yamaha expression pedal in a Roland! Although it SORT of works, the taper of the pot is all wrong, and you get the majority of the action in one small area of the throw. Roland pedals work smoothly through the whole range of motion.

I've never used a 3rd party expression pedal for Roland... I don't know if anyone makes one specifically, do they? Be careful you don't get one with the Yamaha taper on the potentiometer...