I believe from experience that there is no exact way to determine beforehand good spots to advertise SFI business. Everything is all about trial and error. You take your own decisions on where to advertise based on your own experience or the experience of others.
Firstly, what works is what you use and you will not know what works until you have tried it.
Secondly, you can get recommendations from people who used some spots earlier and they do testify that those spots are good.
Thirdly, you can use the recommendations of professional online marketers.
Best wishes!!