I have a bunch of second hand bricks and some are "wheel wells" for the lack of better words.
I would like to find out how I use them for example at bricklink, but then I need to find the name for such a thing first
That is a Vehicle, Mudguard 2 x 4 with Arch Studded (and a darn hard part to figure out at that).
i'm calling those things "fenders" (don't know the "official" name, too).
if found wheel arch 2x4 in the LEGO-shop. this isn't exactly what you were looking for, but looks like a "newer version" - so maybe we should call those pieces "wheel arches" in the future.