I had a really bad sinus headache and went about looking for a Neti pot the next day. I read online that it could be found at a certain Etos in Amsterdam so I went and checked out Etos on Frederik Hendriklaan during lunch break. They did not have a neti pot. I was directed to Kruidvat but they didn’t have it either.
As luck would have it, two women who overheard my conversation with the woman at the store let me know that I could find it at a drug store 200 meters away. She showed me where it was and as she said, I found it there!
So you should be able to find one at any drug store. If not, here is the exact drug store I went to on a map. Hope this helps you and may your sinus be cured and never return.