There is a tour guide who claims to still be able to provide access to the entire market, including the tuna auction - website is here.

Make sure you email for a reservation far enough in advance though - my sister and I planned to use them but they use serious spam filters on their email account and I didn't get a response in time.

Don't know where you're planning on staying, but the Park Hyatt also can arrange a private tour of the market/auction with the hotel's sushi chef - unfortunately, we didn't know that was available and you have to reserve two weeks in advance.

We just ended up going to the market by ourselves - there were other tourists and we caught the end of the tuna auction, since you could see it without actually entering the auction area. There were several Japanese and Western tourists grouped around the doorway. I would have preferred a tour though.