I had all my machining work done on my YB at Scholar, they also supplied, machined, and balanced my pistons, as well as balancing the rest of the bottom end for me. As said, not the cheapest, but as in so many engineering applications, you get what you pay for!
Another vote for scholar. As I think I've mentioned before I've taken some parts that they had done in the past for me to Steve Smith at Vibration Free (just about the top balancing guru in the country) and he said the bits that Scholar had done were as good as he tends to see.