It is chaper specific what markings are used. Those Todosi posted are the "normal" ones.
Those for example are raven guard ones:
I haven't seen specific devastator shoulder pads from
GW or 3rd party ones.
But independant of what markings your chosen chapter uses you have some different options. Paint them on as freehands, try to get hold of some decals (Forgeworld has some additional sheets if your markings are not on the normal
SM sheets) or sculpt them yourself out of
GS.
If you've done the latter you can create a pressmold and recreate them for the rest of your devastators.