You would most likely be looking for the valve seal for the Steam Valve. I replaced mine a while back - got it from Coffee Parts over there <---
When I did it, I replaced the o-rings on the steam arm and the valve at the same time. (Grabbed a group head seal too, but havent needed that yet).
You will also need some food grade hi temperature grease to lube up the valve thread and the o-rings. I bought the tube available from Jaycar.
Link to parts page at Coffee Parts (assuming its the same pre-08 model as mine):
The job is pretty straight forward, but does require removal of the steam valve from the machine.