Aaahhh...Have I?
O.k...To adulterate fen [and analogs] properly you need a ratary evaporator which costs about 10 thousand [US]. It piggy backs the molecules onto inert carriers like lactose, et cetera.
As far as how common they may be...The last bust was Williamson in Big Bear Lake in California. He manufactured about 4 kilos [at least he was busted with that much] with less than a semester of community college under his belt as well as being unemployed and on methadone. So...it is exceedingly easy to do with the right gumption. Fentanyl itself is a 4 step process with the first part being the hardest [and that is not saying much]. The analogs are a bit different but still not that difficult.
As others have said, and I have certainly said before, you will not find it too much because heroin has a much longer duration. you will not find too many junkies keen on shooting up every 45 minutes. The last, and most well known, case of it being sold as heroin was 4 blocks away from where I am posting this. In 91 "Tango and Cash" brand heroin killed scores of people. It was actually 3-meth [potent analog] that had been adulterated as one would heroin. As most know, that still left it about 7000 times too strong. I knew a few who died from it, in fact I would have most likely died from it had I not been in prison when they sold it. You see, there are all kinds of reasons why it isn't too abundant on the street.
You can make it for about 2000 [US] per kilo but the machine necessary to adulterate it properly is a bit more expensive. You would also have to cut it with an astronomical amount of inert carrier which is not a pleasant proposition for most "cooks." The man most often associated with "tango" was Marquardt. He shipped it as is and his man in charge of cutting it, a PCP cook he met in Federal prison, decided it wasn't worth the trouble of laying it correctly. Instead of the 7 years Marquardt had gotten for cooking up crank he got life without parole for the estimated 245 people who died.
I am sure that you will hear of it from time to time but you most likely will not ever see it and perhaps that is a good thing.