Balls on Bricklink 2022-10, $2.00 each, ouch!. An off-brand solution at a much more reasonable price. I've never tried them, but some internet people have had good results.
Electric, Train Speed Regulator 9V Better than batteries. Also, for those of you who may be interested in using the train controllers for wall power, you can use any 12VAC wall adapter with at least 1A output on a 5.5mm plug, e.g. this on Amazon. If you have got a collection of wall warts from random dead electronics in the past, you may already have one of these lying around. Total cost to get the controller & power supply is under $20 (still, in 2022).
Connector wire To go from 9v train speed regulator to power function motors. Sadly out of production, updated the link to brinklink.
Great Ball Pit A regular blog that supports the GBC online communuity; a bit dormant during the pandemic
Revised Directions for the Push Arm GBC, using the power standard instead of a Motor. Likely to be a kit/workshop at Bricks Cascade 2017. Maybe 2018... Parts list for this version.
Forum thread on EuroBricks.com with directions for making some of Akiyuki's builds
Brickworld Ball Pump The 2018 kit directions; a remarkably reliable ball pump. I experienced some occations when it failed to feed, but it never jammed, and presumably would have eventually stirred a ball into the right place. Or maybe I did the directions wrong. A solid beginner pump for people with the required parts.