Interesting problem. Does your car have cruise control? And you're sure the worn part is the metal ball, not the plastic socket that clips onto it? Those act directly on each other, and I would be a little surprised if the plastic wore the metal faster than the metal wore the plastic. The throttle and cruise spools on cars w/cruise often do have some problems with wear of the inner bushing against the spindle, but wear of that linkage ball would be a new one.
Unless you can see very clear erosion of the ball, you may want to first try replacing the plastic ends of the linkage rod from the spool to the throttle lever on the IP, which contains that socket -- that is a common part, same as what is used on 4cyl gasser Volvos, so easy to find good used examples. (Or I have several that I would send you free, for the price of postage.)
If that doesn't solve the issue, I have a few good used non-cruise 7/9 throttle spool assys that I can part with, but I suggest trying fresh sockets first.
Where are you located? (Based on what you drive, somewhere overseas -- UK?
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Welcome to the forum!