What is wrong with the current ones? They only spin when unloaded so are unlikely to accumulate any wear. In any case by your picture it appears that you have as many as will fit minus 1 as is standard practice.

If you can remove 1 & measure it in inch, fractional, my guess is you'd find it measures 1/8th of an inch or approximately 3.18 millimeters or possibly 5/32nd inch or approximately 3.97 millimeters...I'm going from memory here, so you'll need to excuse any inaccuracy. What you need is a digital caliper to actually measure it yourself or take the ball to your local bike shop and have the mechanic do it for you under the promise that you are going to buy a handful at ten cents a piece. Then buy as many as will fit, minus one.