On a marketplace investigation doco on YouTube they just broke open the capsules and sniffed them. Bad smell means it has oxidation going on and that means it’s bad quality. You could buy a reputable expensive brand but if they haven’t been stored well they could still be degraded.
Algae oil is arguably the best way to get omega 3 (including DHA+EPA) and is vegetarian. Fish contain omega 3 because they eat algae so why not get the omega 3 straight from the source? Algae oil is also less likely to be contaminated with heavy metals. You can find algae oil supplements online easily and probably in most large health food shops. I’m in the UK so not sure what brands to suggest for you.