When Android Auto was first introduced, your only option for using it was to buy a brand new car. But soon enough, third-party head units (like the lovely Sony one pictured above) started showing up. Now that you can run the full Android Auto interface on your phone, you don't strictly need a compatible head unit, but they're getting cheaper anyways.
Now you can get the Sony XAV-AX100 and XAV-AX200 head units for $50 off the usual prices. The AX100 was released last year, but is still an impressive unit. It has a 6.4" touch screen, full support for both Apple CarPlay and Android Auto, a 10-band equalizer, and a rotary dial for distraction-free functionality.
The AX200 is a bit fancier. It's pretty similar to the head unit it replaces, but with the added option of a built-in DVD player. So when you get bored during bumper-to-bumper traffic, you can stick in your favorite movie. Just don't do it while driving.
You can buy both units from the links below.