I'm in the same boat as you with regards to whether EPA/DHA has a bigger effect than ALA, but I was convinced enough to try to find some when I became vegetarian last year.
If you google "algal dha together" you'll find what I'm taking - meeting your criteria of vegetarian (vegan), eco-friendly and health-friendly (with aforementioned uncertainty)
ALA can also be found in flaxseed, soy/tofu, walnut and pumpkin, so you needn't stick to seaweed if you only want ALA.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
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