It's been a couple years since I've needed one, but if memory serves correctly, retroreflectors won't flip an image around when you look at it (unlike a conventional mirror). They are kind of difficult to make so they cost a lot of money.. the more you spend, the better the reflection will be. Obviously, you want the image to come right back on itself and not at an angle. We tested this by shining an IR source (laser pen) directly at it and looked at where the image (red dot) veered off to. We first made sure the pen and retroreflector were in alignment. I believe Newport or Thorlabs carries very high quality retroreflectors...
Good Luck!
--Username