best thing to do would be to buy one through the club if they do that.
otherwise just a standard white oak should be fine.
i wouldnt go for a niten unless you are doing partner work with someone else who has a niten bokken as they are thinner and chunky bokto bash dents into them. though they are grand for lone work as they are more sword shaped.
get a nice tough wood. and avoid suburi bokto as they are too big.
ninecircles offer a good range too.