I have a cedar tree that fits well in the Chelsea style boots, not made specificaly for boots, but it works fine...$12.95 a pair. Similar to below, except the heel is cedar as well with a leather rope strap.
I agree with Chris. No need for boot trees for ankle high boots - regular shoe trees will do the trick. Plus, the only place I've seen trees for these kinds of boots are at custom bootmakers like Lobb and Edward Green.