I currently use directbet.eu but they only offer a few racetracks to bet on and there are very few north american tracks to wager on and you only can bet w/p/s. If there are any other sites that offer better horse racing please post names of sites thanks.
There doesn't seem to be any other good site offering Horse racing. Maybe you can try on one of the fiat sites .
Now that neteller accepts deposits with bitcoin, he can use any site that accepts neteller. Not as good as using bitcoin directly but it's better than nothing. I recommend this and betfair. You can bet for and against results there which is very useful. Especially if you have bonuses you want to withdraw in other sites with the same bets.