Here are two sources for this part:
https://www.cpi-va.us/store/p/earth-sky-planter-gwxkp
https://leitner-wise.com/store/G2-Heavy-Duty-Buffer-Retainer-Now-Shipping-p141104163
https://www.vktrind.com/product/heavy-duty-buffer-retainer/
Yes, I know you can use a small flat screwdriver to get in there with a standard detent but doing below is much easier.
