the end green pieces by where the wheels would be
If I understand your question correctly, I think your looking for this vehicle mudguard piece?
Lego refers to the piece as "Brick 2X4x1 W. Screen", but others may use the Bricklink description:"Vehicle, Mudguard 4 x 2 1/2 x 2 with Arch Round ". With that in mind, it's Design ID is: 50745 which you can enter into the search field to the right of the Lego Digitial Designer Logo on the Bricks tab. That will show the piece.
If your trying to find it manually, it's in the group with this "tail" piece icon.
Hope this helps.