Need help identifying or locating pieces - 4-stud T-shaped plates
I can't speak for other brands, but I'm sure that there is no such LEGO element.
This would be a plate, but the Bricklink category has nothing exactly like this design.
The closest would be the Plate 2 x 2 Corner or Plate 3 x 3 Cross, but the first requires an additional plate, which could prove unfeasible due to the missing structural bonds, and the latter has an extra plate (which could be cut away if needed, even though that would be considered a bad choice by many).

Edit
As jpa noted below, if these parts should serve as structural support, the required strength could be achieved by attaching a Plate 1 X 3 and a Plate 1 X 1 together by their sides. These LEGO bricks are made of ABS, therefore the suitable binding agents would be acetone, cyanoacrylate (superglue), or epoxy.