What you can do is get some tsuka-ito off ebay or so, shouldn't cost you more than 20$ at most, and then wrap your bokuto's handle the same way a sword is. It isn't too difficult, it's quite fun, and the final result looks spectacular.
I'm with Bruce. I've always used beeswax and orange oil on my bokken. Well, on the ones that I've bothered to worry about putting anything on. Might I suggest perhaps working harder on your tenouchi and shibori as a way to prevent slipping?