My favorite tickling tool is a pasta spoon with nice, rounded teeth. Try to find one with a little bend in the handle. Of course, you'll want to use baby oil on her soles. Also, a device called a Whartenburg (I think that's how it's spelled.) wheel is out there on the internet. Look it up and see what you think. Ball point pens are also effective tickle tools. Keep a damp cloth or some wipes nearby. Write on her soles, and let her know that if she moves and causes you to mess up, you'll clean her off and start over. This adds tension and makes her nervous and more ticklish. Also, I've used a soap brush filled with baby oil. You can find this at Walmart or the grocery store, usually located near the dish cloths and towels. It's normally a round brush with a reservoir in the handle.
Regardless of what you decide to use, I'd like to hear about the outcome. Be sure to post about it!