My favorite is a free one borrowed from another TUGger. Perhaps someone will mail you one- or pass one to you in Hawaii as happened to us last January.
The road to Hana has been improved so much in recent years that it's no where near the ordeal that driving to Hana was a few years ago. Turn-outs are marked, the pavement doesn't have craters and it's even got lane defining paint. The road still has a lot of curves and a CD will tell you about upcoming attractions and where to park to see 'em, but (imo) not worth paying much.
Enjoy a very full day doing it.
Jim