We will visit Amsterdam next March and I plan to rent a bike for day. Do they include helmets with the rental? I watched several videos of bicycles in Amsterdam and I saw nobody wearing a helmet.
There are people who cycle with helmets in the Netherlands. But they are rare and rental bikes do not come with helmets at all.
(And to prove the rule, I found an exception, a site where with a bike you can rent a helmet, not in the city though.)
When we see a group of more than three people with helmets who are not on racing bikes, we say to each other 'tourists'. But people who race or ride mountain bikes off road often wear helmets and slowly more commuters also use helmets.
If you own helmets, nothing stops you bringing them and using them in the Netherlands. You will look like tourists but hey, that is what you are.
A quick internet search did show no adult helmet rent, I did find child size helmets for rent with special bike seats and child carrying bikes.
There are helmets for rent for events, like triathlons, rental prices may be more than the rent of a bike for a day. Site 1, Site 2, I found these sites online and do not know anything about their quality (and certainly have no link with them.)
For a longer stay/rental you can buy a helmet locally, but I think for a one day rental that is unlikely.
To search for a helmet to rent, with or without a bike, search on the Dutch name:
fietshelm huren (name of city)
I would do like the locals do, forget the helmet.
(I am Dutch, I do not wear a helmet but I do cycle daily and have done so most of my life.)
PS, out of personal experience:
Leave the city for the best cycling. You can rent bicycles all over the country and riding in the countryside is more enjoyable than in Amsterdam. While it is the worlds best cycling capital city, it is below standard for the country. And it gets very crowded on the cycle lanes in peak hour.