Do you know who would sell the single-frequency macchine you describe?
No. All the machines we worked with were prototypes made by my father. But again, he does not make them commercially.
I have a friend though that is going to electronics school. He is talking about making one as a school project then starting to build them. I found some schematics for him, but I think they are both 3 frequency. These could be easily adjusted though to correct the other two frequencies to the proper frequencies so they work just as well as the 666hz.