Well right now there are a guaranteed 4 ways to get Etheric Light every week. This would be from Level 34 and 35 Prison of Elders and the Silver and Gold tier packages from the Trials of Osiris. Once the Iron Banner starts again weave another two guaranteed ways by hitting rank 3 and 5. I've seen people get it from Nightfalls, but it's pretty inconsistent as a drop. So overall we have Nightfall, Iron Banner (2), Trials of Osiris (2), and Prison of Elders (2). I think that's good enough, if you have more than one character then you have plenty of opportunities. If you only have one character then you probably don't have a ton of time to play, so hitting 34 wouldn't be a huge priority.