Depends on your budget and how big your lathe is. The dog's wotsits is the Holdridge (lever operated) but these are a little bit rare and can be expensive. Another to look for is the Habit Turnaround....no, it really is called that! This is a geared type.
There are other branded ones out there but the above mentioned are those I have seen.
There are two styles of ball turner, vertical axis and horizontal axis. For toolpost mounting, a horizontal axis one is a little easier. There are designs that use a standard boring head with straight shank in this manner. I am fairly sure DoubleBoost showed one this year, using a reground endmill as the cutting bit.