We had a lot of trouble finding it just like you! We tried hardware stores, calling fencing places... We eventually gave in and bought from Amazon when it became available (it took a lot of digging to find the right size and dimensions at the right time).

Here is what we bought, though it looks like it is currently unavailable. https://smile.amazon.com/Jackson-Wire-11033824-Vinyl-Hardware/dp/B002LZP6YC

I will say, the vinyl made a difference and was worth it.