Hi franky-z I just typed in Makpung Bull racing in Bali and there were heaps of site for it.
Bull Racing is held from July to October. The bull races ("Mekepung") are thrilling contests between teams of two young bulls harnessed to a chariot, which is ridden by a jockey who holds onto the bull's tails as they career down a 2km race course! Winning is everything, and for the winning jockey it is a ticket to stardom, and perhaps a chance to become national champion. The bulls are treated with great care and are fed elaborate diets containing all sorts of wonderful potions and secret ingredients. This is a must-see for the whole family. The competition are held to determine the champions from either side of the river, who will then complete against each other at the annual Governors Cup race held at the end of the rice harvest in mid summer. The bull race is held twice a year, July and October, every Sunday morning.
One site did say to check with a tour guide as some Sundays they don't have it.