The two exchange traded binaries are the BVZ and BSZ. These represent the VIX and the SPX and are traded on the CBOE. I would agree with what Deltahedge posted. The markets are relatively wide and many brokerage houses don't allow you to trade them, mainly because their risk programs have problems dealing with them. Due to their limited volume, I guess they don't want to re-write their programs.
That being said, I still trade these a fair amount. If you trade Iron Condors on these products these can be very good. For instance say you are going to sell the 15/16 put spread in the VIX. Currently you can get about .20 for it (both bids). If you are able to sell the BVZ 15 put binary for .20 you spend less in commission and you don't lose money unless the VIX settles below 15. With the put spread you would lose below 15.80
There are also Arb opportunities between the binaries and the spreads in the underlying indexes. For instance in the above example if you buy the 15/16 put spread in the VIX for .20 and sell the 15 BVZ put for .20 for a net of 0, you would make money between 16 and 15 and have no risk.