A member here at ASC named Dynamo is making these valves on an "order" basis. See this thread (originally about the dangerwerx valve. Read the first page, and then the last page):
http://www.airsoftcanada.com/showthread.php?t=147346
However, he posted about a month ago that he's been swamped with real work, and hasn't been able to produce any lately. You could keep an eye on the thread there.
I picked up one of Dynamo's, and my friend is using it in his MP9, and it's shooting sub-350. (I can't remember the actual FPS though).
Alternatively, the Dangerwerx valve mentioned in that thread will also supposedly lower the FPS to sub-350.