What salt do you need for a ice cream maker?

Rock salt is cheapest, table salt works just as good. These will cool the salt and ice to about 10F.

There are other salts that will get the salt and ice mixture colder, but they are generally more expensive and harder to get in quantity.

If you want to experiment (and will be very very careful never to accidentally contaminate the ice cream mixture) you could test some of the "Ice Melter" products used on sidewalks in winter, some of these should cool the salt and ice somewhat below 0F. But these could be toxic!