Do I need a downrod or special bracket for a ceiling fan mount on a slanted ceiling?
Some fans are designed to be installed without a down rod, where the fan housing mounts rigidly to the base plate (which in turn mounts to the electrical box). These are more rare lately since the stubby downrod approach tends to be simpler to install (and probably cheaper to manufacture). 

Whether those fans' housings are robust enough to handle their weight at an angle will depend on the particular fan. I would expect that the fan's instructions would specify whether this is an option for low-profile mounting. It's not going to be a matter of special brackets, but housing design.