GW Silver Skulls pads work fine for PH Iron Warriors, too.
There are several Legions which remain the same and are available in official minis - Ultramarines, Dark Angels, Space Wolves (and therefore, Luna Wolves too - one of the Great Companies' symbols is perfect!), Iron Hands, Blood Angels, Salamanders and Imperial Fists.
The Death Guard and Thousands Sons can be converted from the Red Scorpions Librarian Honour Guard:
but given the expense, probably better to freehand!
I freehanded a World Eaters banner and found it remarkably easy. I will try it out on shoulderpads and let you know if I can make a tutorial.