Surely how you go about finding people very much depends on who you want to interview and about what?
For example, I'm interested in talking to horse owners - I'm actually doing a mixed methods study and will be recruiting from my questionnaire, but assuming I wasn't and I just wanted to do interviews, I would probably put adverts on social media and in equine magazines.
If I wanted something specific, say for example I was interested in talking to breeders, I'd research breed registers and see if I could get the advert sent either directly to their mailing lists, or again up on their websites, or sharing to specialists facebook pages etc.
Perhaps if you give us a bit more detail about the problems you're having?