I thought they were all 1/4" in those Grimecas............maybe they ARE 7/32". They come pre-packaged from Akron Moped in packs of twenty balls, but I usually just pickthem up at Handy Bikes when I'm in Columbus. They take 11 balls in the rear wheel,i'm pretty sure. If the front axle is smaller, it takes 10 balls.
Call Ike or ChrisMWH to double-check. don-ohio