IIRC we have no clear evidence what Seleucid soldiers, let alone cataphracts, actually looked like. The only contemporary depictions of Seleucid equipment may be spoils of war shown on the Pergamon friezes – but even that's guess work, for we cannot be sure if the Pergamon monument was built to remember a triumph over the Seleucids (most likely that of Eumenes, as a Roman ally, at Magnesia).
However, it's a widely accepted view, so you may as well follow that route. Mithridates1 has provided a very good start.