Metric was our first choice. I was suprised too that we couldn't find a metric to fit. Of course the only ones we had to try was what was in Auto Zone'* inventory. It could be that we didn't have the right thread pitch. However the 1/4-28 did fit good, it threaded in all the way, no binding, and wasn't noticably loose.
The moral of this story is that the standard pullers don't come with the right bolts, the ones for our balancers are small and very fine threaded, and good luck locating the proper ones on a Sunday afternoon. I want to reinforce the advice I had here, (and obviously didn't follow), to not leave the parts store without test fitting the puller to the balancer.