The best place to source a new cylinder stop would be S&W parts department, IMO. Ball width is approx. .100".
Is there too much play or gauge in the stop, or is the stop not holding? If it is not holding properly, inadequate stop spring tension can sometimes be an issue. You might check to see that the stop slots in the cylinder are clean, and that the current stop is rising high enough into the cylinder slots to hold properly.
S&W oversize stop part #'s
stainless/oversize: part # 07454
blue steel/oversize: part# 0535702
Added: S&W MIM stops are part #21508. The oversized cyl. stops are now listed as "obsolete". (like some of us

)
If you need to verify a part number........PDF's of the parts lists (the last one I have) are available under the "Notable Thread Index" at the top of the "Smithing" page.
Go to "PDF's of the Parts Lists". Auto and revolver lists are available.
The other alternative would be the
Power Custom oversized cylinder stop. They are very high quality, but the issue with them is they are oversized in almost every dimension, not just the width of the ball. This makes them more difficult to fit. I have seen them so wide, in fact, that the ball would not pass through the frame slot. The ball width of these stops is usually over .110" in my experience.
Whichever part you choose, it will likely need fitting to the gun to work properly.
Carter