Hey, B and I found these steel things that you can slide on your fingertips - they are rings with two "spikes" or "claws". Can't find 'em now. If anyone knows a source, we would appreciate a link! Thanks!
Plastic guitar finger picks work well. Banjo picks are metal and sharp. They need to be used extremely lightly, but if used correctly they can drive a tickle victim insane, especially very lightly around the sides & ribs
I've used plastic banjo picks and they were devastating Great for tickling feet, backs of knees, between toes, basically everywhere light scratching can be used!