[1] https://news.ycombinator.com/item?id=16600388
As for making your own, it depends on how good you want it to be. For example, the CommaAI is simple to do even from scratch but not super cutting edge. On the other hand theres companys out there using additional radars and Lidar which increases the cost and complexity. But in theory you'd just need to mount a machine in the trunk of your car, hook it up to your cars computer (have a car capable of being controlled solely via computer), hook up any additional sensors. Then write the software that can fuse all of the incoming data and use it to send messages to the car.
If this is something you're interested in learning about you can check out Udacity's SDC Nanodegree
Everything's open source, and if you want to build your own self-driving car, probably start there. They're selling more or less all of the parts you need, albeit in a very DIY fashion.
I think the most recent part they've started selling is something to more directly interface with the car's accelerator pedals, for example.
The rest is a matter of scaling up, both in car size (easy) and in software (lots of work)