I don't know of a tick repellent. All the sudden I am having ticks here--strange season for them! I am spraying my horses' legs and lower body with oil-base fly spray, hoping it will deter the little monsters. Mine seem to be getting the ticks from the grass, and not from the trees, as so far one has been under the jaw and one beside the mouth. I check the ends of the tails, too, as I find a tick there sometimes. None on their backs, manes, or ears--thank goodness!
This season seems to be one of constant vigilance here. Besides watching for the ticks, I've been removing bot eggs and those little whirlie grass seeds that screw into the skin. I have not had bots or those grass seeds in several years.