Only reason I said vanguard marks is because you're limited to earning 100 per week, so you could only buy one each week and then nothing else from the vanguard, meaning you'd have to think about whether to do it or not. Motes of light is a good suggestion, though I know people who already have at least 200 in their inventory due to not having a reason to use them. Would take longer to earn if you didn't have them already though.