The answer is no retail brokers offer them, at least CBOE couldn't tell me any that did and volume reflects that. Nadex and Cantor Exchange are the only CFTC regulated binary exchanges that a retail investor can realistically access, and Cantor only offer forex while Nadex does offer S&P500 but not VIX.Binaries on the VIX are traded on the CBOE. They used to offer binaries on the SPX, but that product was delisted several months ago. Unfortunately there are a limited number of brokers that offer trading on this product at the CBOE. Not sure who currently offers them, although most front end products such as Redi do show them.
You are really better off replicating a binary option through a vertical. This will give you a similar product and you will give up less edge.
One other note, the CBOE binary options use the "Special Opening Quotation (SOQ) of VIX calculated from the sequence of opening prices of the options used to calculate the index on the settlement date." which is a funky way of calculating the VIX level that can result in a VIX settlement that is quite different from the opening VIX price and can even be higher or lower than any VIX quote all day. This may make sense in some situations when they do it for the S&P 500 regular index options, but for binary options it's pretty senseless and combined with the lack of retail brokers it ensures these products will never catch on.
there won't be any merit for the broker which ensured best execution and spreads for him..