A mimic octopus will be very difficult to find- I don’t know of any suppliers that carry them regularly when they are available, or even which season is the best to look for them. I’ve seen a couple of online stores that have waiting lists for them, which I was able to find with a quick google search, but none that I’ve actually done business with and can personally recommend.
What size is your tank? There are other species that shouldn’t be as difficult to come by, especially if you wait until late spring!