I don't know of any in stock, but they come up on gunbroker every once in a while. There was a guy on HKPro selling extended barrels a while back that you could have threaded, but I just did a quick search and couldn't find him. Might want to ask over there if you haven't, I'm sure someone will know.
You may already know this, but the P7 suppresses poorly. It is quieter than not having a suppressor, but much louder than other 9mms. I've been told it is because of the piston system, and don't know if that is true, but I do know that compared to a Glock and Sig with the same can the P7 didn't sound great.