You may want to take a look at dronecode.org, or their supported projects. Especially, PX4 (which was also mentioned in another answer):
Getting Started with PX4 Autopilot guide
PX4's Follow-Me Flight Mode
MAVLink's Follow Me plugin
You may also find some interest in some other open sourced drone projects, e.g. ArduPilot's Follow Me Mode. Another one ...
The PX4 open-source flight stack has made some progress on various forms of computer vision. While not exactly turnkey, it’s an active area of development and you absolutely can buy hardware that supports it as long as you’re comfortable getting your hands dirty setting up the software.