How to find the plastic rolling clip for window regulator arm. 1994 BMW 318i

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I have ordered and replaced the window regulator clips on my 1994 BMW 318i The window would not roll up or down without falling off track. The clips were old and worn down. So this fixed part of my problem. I now see that the plastic piece connecting the third arm portion of the regulator is also worn causing it to pop off when trying to roll the window as it reaches all the way up, and it falls off track again. The weight of the window pulls at the arm and it pops out! Where can I find the plastic rolling piece or bushing without replacing the whole window regulator? Thanks! It's a 1994 BMW 318i