Hello, I was wondering what this is called. I need them for my front fenders. Also does anyone know where to buy one? 2018 F150.

 

I found similar flares that include those and according to the installation instructions they are simply called inner piece. Fits inside the edge of the fender. You may need to contact the manufacturer of your flares and see if they can help you out.

Here is the installation PDF for a set of similar flares which include similar inside support pieces.
https://www.cjponyparts.com/media/im...ll_2092602.pdf