While I don't have any ideas regarding niches I just use a technique that was posted here.
I would look for the standard disclaimer text that Amazon requires of its affiliates, I would then skim through the results to see any interesting sites.
But if I don't have time to do that, I would scrape the domains and then filter out big publisher domains like Forbes, Fortune, Chron.com and I would plugin the domains into SEMrush and outcomes organic traffic estimates.
I would then zero in on obvious affiliate sites that have a decent organic traffic estimate and I'd look at their keywords that have low difficulty ratings.
This is how I zero in on low-hanging fruit if I don't have any niche ideas.