Can you buy a Mexican VW beetle?

If you want to drive it in Mexico, yes.

There used to be a company called Beetlemex. They went out of business many years ago, but what Beetlemex did was to take a Bug that had been registered in the US, drive it to Mexico and take it apart. They would take the chassis, rebuild it to be like new, then put all the parts off a Mexican Beetle on it. They'd then install the two number plates from the old body onto the new one and bring the car back to the US. I wouldn't recommend it unless you live very close to Mexico, because you'll need to go there to get some of the parts you'll need.