They come on the M&P15x and possibly other models. I don't think they are sold separately. You would have to wait and find someone selling one they didn't want, which is rare.
You can check the on-line auction and sale sites. I bought a M&P15A with the Troy rear because I wanted a BUIS with front sight tower and a red dot. I suspect most users bought the 15A and 15X for the same reason, so the sight itself will be extremely rare. You will probably end up with one that is Troy marked.
Thanks guys, I figured it was probably going to be a difficult find so I just purchased a standard troy rear BUIS. I would still consider purchasing if one came along though. I am also in the market for the smith flash hider that they use on the magpul edition as well.