So-called "left-hander-nibs" do not exist. Some nib types however are very suitable for left-handers: Ly-nibs, which are oblique to the left, ball-pointed nibs, which do not stick on the paper due to their construction and ellbow-nibs that allow emphasis in Copperplate f. e.
With some practice left-handers can use almost all types of nibs, but for the right-oblique broad nibs is to say that the use of these nibs changes the way of holding the tool in a very unnatural way.