You're bringing up the subject all wrong. If you talk about tickling and how much you like it, 90 percent of girls out there are going to react uncomfortably at first. I knew a girl in college who tickled every guy she could get her hands on. The minute I brought up the fact that I thought it was hot the way she tickled guys, not only did the tickling stop, but she became very self-conscious around me. Think about it like this. Girls walk around every day being hit on, leered at, and pursued by guys who want to sleep with them. The last thing a girl wants to know is that her playful habit of poking her guy friends in the side is giving some random dude in the room a boner.
Your best bet is to introduce tickling into your relationship with your female friends and romantic interests in a non-threatening, natural way. Next time she accidentally brushes your side with her hand, jerk away, even if it doesn't tickle. She's almost certain to ask if you're ticklish. At that point you're good to go. No matter what you say, she'll probably try to tickle you again, at which point you can tickle her. If she doesn't try again, you can still tickle her by saying, "Why, are YOU ticklish?" The ice will be broken and you can escalate the tickling in the future.
The point is, the last thing you want the girl to think is that you're some creepy guy who is trying to use her to give yourself a boner. That's why you shouldn't bring up the fetish, or your interest in tickling in general. What you have to do is create a situation where tickling comes up naturally, and slowly incorporate it into your relationship with your female friend/girlfriend/etc.