This bugged out even me. From what I've found out this adaptor has been out of stock for a very long time from the official oneplus store (There have been several complaints about it as this is something that is prone to breaking and getting lost).
In order to get them offline I tried getting in touch with the oneplus experience store. I tried calling them multiple times they but they never picked up. So then I went to the store personally only to find it was out of stock, and would take at least 2 weeks for them to get it back in stock(If that happens).
Then I tried my luck with multiple local stores but none of them had it.
Now, getting to the important part. I've been researching about it for a while now and might've finally found an adaptor which would most likely perform on par when compared to the stock oneplus adaptor and is of good quality. It's the
Mi adaptor.
For Indian costumers: We can use Sharesave App to get the adaptor.
https://sharesave.mi.com/products/4420?from=copyLink
(While placing the order remember to pair with other people who are waiting to friend so that you both can avail the discount).
For other countries:
https://m-in.gearbest.com/chargers-cables/pp_780293.html