What particular Mazda, and what budget?

I don't recommend getting secondhand automatics, especially not Mazdas.

For one, you never know how well the previous owner cared for it, and AT problems usually have one solution... buy another (surplus) transmission to replace the broken one...

Also, old Mazda ATs aren't fuel efficient, and are prone to the odd breakdown. Unless you're very sure of the car you're buying, better look at something else.