Water level indicator for overhead tank in an apartment in Kumbakonam, Tamil Nadu : Need the name and vendor contacts for the visual cue model

I am living in an apartment near Kumbakonam, Tamil Nadu. For our overhead tank, we wish to install a water level indicator. There are automatic electronic water level indicators available in the market. We are not looking for them. We require a simple water level indicator (a Glass tube if i am right, is fitted adjacent to the water tank). No electronics.No sensors. We wish to see the water level from outside instead of opening the tank.

Now, I tried an internet search for this requirement,  but no luck. I wish to know the correct name of the product. Contact details of dealer/ vendor in Tamilnadu will be appreciated!

R Balaji