Hi Elza
I just saw your posting enquiring about Kalchatti. I am so glad to help you out in this, unfortunately not from Chennai, but from Palakkad, Kerala.
Yes. I was in Palakkad in August last, visited this place after many many years (my father belongs to one of the heritage gramams of Palakkad), and happened to stay here for a week this time.
In Palakkad, there is a heritage gramam called Kalpathy. There, there is a street named Kal Chatti Street. I was curious to know why is it so named........ I was told that once upon a time, Kalchattis were sold in the road, the name derived that way...... But now there is no shop selling. But the road side shops will be opened during temple festival in Kalpathy.
The temple festival has started from 1st November and will go on for 2 weeks with a famous Car Festival concluding the event. So there should be shops selling Kalchattis. You may try a visit and buy from there. But I was also told that otherwise also Kalchattis are sold somewhere in the Angadi there in Palakkad. I could not get one as I cannot carry due to heaviness for overseas journey

.
Enjoy your trip and buy one and have nice tasty food made in the kalchatti and share the experience.
Good Luck.
Shanti