George Hotz's comma.ai started as something he built in his garage. (From the original video, you could see off-the-shelf stuff like AmazonBasics USB hubs and all.)

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.