The only time I've ever been successful with this was with massage therapists. I used to get massages with my father in Atlantic City back in the 1980s, and it was humilating, because I was so ticklish, that the massage therapist could hardly touch me without my cracking up. In the course of conversation, they used to say that they dealt with very ticklish clients, had massaged ticklish family members, or themselves were ticklish when massaged. Of course, all the massage therapists were women, so it was nice to hear, even back then, before I realized this was a fetish.
More recently, I have gone to a massage therapist named Kandy with my best friend. Kandy is hot. I've seen her several times, and she makes you sign a waiver form saying that any form of sexual harrassment, the session is over. So, while I couldnt exactly ask her "Kandy, are you ticklish", for obvious reasons. One day, while getting a massage, I did pose the more general question of whether she dealt with clients who said massage felt ticklish, and she said most definitely yes, that she had also massaged clients, and family members who were ticklish, and that massage can definitely tickle.
A massage therapist seems to be a great place to bring up the subject of tickling, as to whether it can tickle or not, and, it has worked for me on a couple of occasions.
Mitch