Cistern

The museum's web site has a Visit button near the top right which leads to info on buying tickets online.

It’s in Turkish

There is a button to switch to English at the top of the page.
I found I had to switch to English again after clicking the link to buy tickets, then I clicked on the Museum link, and I believe I had to select English again.
End Result: https://www.passo.com.tr/en/event-group/serefiye-sarnici-istanbul-muzeleri/136613

Thank you!!

I followed the link tried several times, still can’t get thru. Any other suggestions for tickets purchase

Use the camera function of google translate to decipher the website in Turkish. It’s caveman but it works.

This website may be helpful. The actual name of the Basilica Cistern is Yerebatan which is indicated by one of the links mentioned above. This particular link opens in English.

https://yerebatan.com/en/