In Australia we can buy omega 3 fish oil at any health food store and over the counter at a pharmacy. Like anything highly recommended by the media, it's expensive.
We tried it with difficult child 3 - no difference. Same story with difficult child 3's autistic young friend - his mother had him on fish oil as an alternative to stims (because she has a 'friend' who keeps telling her we shouldn't drug our kids - 'friend' has door-to-door business in alternative supplements). And difficult child 3's autistic young friend - no difference there, either.
But other kids we know - their parents reckon it works for them.
If a therapeutic claim is being made and can be substantiated by research, then FDA will be involved (if it's anything like Australia's TGA). But if it's advertised as a supplement, or food extract, no therapeutic claim can be made in detail. You will get broad statements and anecdotal testimonials, but nothing more specific. They cannot advertise, "This will make autistic kids better" because that can't be proven. But they CAN say, "... as used in treatment of ADHD and autism." Because THAT statement is actually not making a therapeutic claim.
It's like a lot of things. It probably will do no harm to try it and if you think there's an improvement, then GREAT! But it's not anything like a certainty.
Marg