You can't buy a fully-autonomous car right now. The best you can do is purchase a car with some sort of driving assist that feels pretty autonomous, but can't replace a human driver - like Tesla's Autopilot. Clearing people still don't understand that part, including one dickhead who was caught sitting in the passenger seat while his Tesla drove down the M1 unsupervised.
In May or last year, while driving at 40mph down the motorway, Bhavesh Patel figured it would be a good idea to switch on Autopilot and jump over to the passenger seat so he could have get some proper relaxation - leaving the steering wheel and pedals completely untended. That's when a passer by noticed what was going on, and caught the incident on camera.
After an investigation Patel ended up pleading guilty to dangerous driving, and has been given a pretty hefty punishment including 100 hours of unpaid work, £1,800 fine, and an 18 month driving ban. He acknowledged that what he had done was "silly" lamenting that he was the "unlucky one who got caught", implying that he knew of other Tesla drivers who do the same sort of thing.
Tesla has always insisted that Autopilot is never intended to be a fully-autonomous system, and requires "fully-attentive driver". Though the name Autopilot can give off the impression that the car is capable of driving itself, to the point where the company has been asked to rename it to avoid misleading customers. Tesla's official line is that there isn't any confusion because “just as in an aeroplane, when used properly, Autopilot reduces driver workload and provides an added layer of safety when compared to purely manual driving.”
But you can never underestimate the power of human beings to get things wrong, intentionally or not. Most people aren't pilots with the knowledge of what a plane's autopilot system actually is, and when they see that word they tend to assume it means human intervention isn't necessary. Sensible and intelligent people won't turn on a Tesla's Autopilot so they can have a nap, or watch Harry Potter, but we're not all sensible and intelligent.