Every faction besides engineers will have a purple(epic) flag that you can make at their respective purple bench.
It costs 200 scrap metal and 60 copper.
This one is actually harder than manufacturing the legendary part in my opinion. It takes awhile to level up to a workbench that will make the flag. I recommend using the blue flags that you can manufacture at the blue benches to speed up the leveling process if you are in a hurry.