That's about it. It's possible you might be able to get some by re-playing the House of Wolves quests through the Abandoned Quests Kiosk, but I could very definitely be wrong about that. You can trade Etheric Light to Petra for 150 rep, so you could get some people and run the 34 and 35 Prison of Elders to get one EL each, on your first run per week. Also, if your House Judgement rank is high enough, Variks has a package for you every week with ammo & materials that sometimes has EL in it as well.