Release they do sell special balls that are manufactured for Hickory play at swap meets that occur at every tournament (you can buy almost anything related to Hickory golf at these swap meets) but there is no requirement to play them. I've seen Top Flites, Srixon, Pinnacle, Titleist, Taylormade, etc played with Hickory clubs.
The clubs themselves are not USGA conforming either. A lot of these guys make their own clubs, but there are some players who mass produce (Tad Moore comes to mind http://www.tadmoore.com/ ) clubs for anyone interested. Louisville golf also continues to make Persimmon clubs if that is your thing as well.
My brother, who is scratch with Hickory clubs, started making his own clubs a few years ago, because he didn't like some of the clubs he found that others were making. My father started him in this whole thing about 10 years ago when he got interested in it. My father has an entire wood shop in his garage devoted to making and selling hickory clubs. I think my Dad has lost interest though, as he's 81 now, and not in the best of physical shape, so with the decline of his health, so has his interest waned in anything golf. I think he only plays once a week now, as he just can't handle 18 holes of golf physically anymore.