Instead of the bull-horn specific brake levers, which mount in the end of the bars, you can also use standard drop-bar brake levers, like a cheap pair of Tektros. Most bullhorn bars have enough curvature so that the levers have enough clearance from the bars to give them room to operate before touching the bar, although with flatter bullhorns, without much of a hooked end, then this will be a problem. The other problem is routing of the brake cable, which comes out of the top of the lever, and so this would then be going forwards. I solved this by filing away some of the brake lever body to allow the cale to exit a little differently, then also using a "flexible noodle" designed for V-brakes to make the tight turn to direct the cable under the bar tape; this causes a bit more friction in the housing, so you might need to increase the tension in the brake's springs, if that is possible with your model of brake.

You could also go for an integrated brake/shifter, there is a model called the Versa-8 that is similar to a Shimano STI, but is designed to operate an Alfine or Nexus 8-speed hub (there is now another model for an Alfine-11 also). Again, you would need a bull-horn with a sufficient amount of bend at the end to mount these and have sufficient range for the brae lever.