Start looking! It took me over 5 years to find one that I was willing to chop on as they all seem to be too nice and collectible. This was the only one I found!
As an FYI, the extended tube is for a Browning BPS. It has the same threads as the Belgian Auto-5s. The Auto-5 clones and Japanese guns are different. If I were limited to a clone or Miroku gun I'd probably forego an extended tube altogether and NFA tax stamp it.
A buddy of mine scored a Remington clone at an auction years ago for dirt cheap. I don't think he wants to sell it though.