Haunt Master products makes them (flip-flop units), just checked they're site 7 they have sold out for the year.......
Unless you can get by with a manual one. Tape 2 light fixture fader knobs face to face.......
I have a fader unit that has two outlets. As one outlet brightens, the other dims. It cycles automatically but the speed can be adjusted. I use it for a pulsing light in a coffin. If you think this might work for a Pepper's Ghost, I'll try to find the manufacturer. Alternatively, I have a DIY light pulsing hack that might work. It has an 8 second dim-to-bright cycle, probably too fast for your needs.
Edit: I found this unit - it looks a lot like the one I have:
This is not the best cotroller out there but I made it with things I had around the garage. At the time, there was a tremendous lack of funds. Otaku will have a much more elegant solution. The one I made has been working for several years now and if I ever have a problem with it, I will convert to one of the electronic units. But here is my "Crank Fader"
Thanks for all the help gang.
Bad news on the fader that Digital SL sold, they don't have them anymore, as the supplier stopped making them.
Still looking for a ready made, but as a back-up , I did find a dimmer switch that I can do the linkage set-up to.