I'm trying to find TM90 Substitute. I understand that it is found on one of the Mirage Islands. I also understand that the Mirage Islands that you can visit are random and change based on Streetpass as well as the online passersby that you meet. However, I don't know how often these change or if there is there is anything to do to get certain islands to appear.
I haven't been looking for very long, but it seems like I keep seeing the same islands over and over, even after trying for a few days.