Mai Sadak, Chandni Chowk also have some shops selling architectural stuff.
If you want you can go to Nai Sarak, there is a shop by name 'GP stationers', I am also an architecture student, I bought everything fron there only. You'll find everything on that shop. It's a popular shop in Nai Sarak.
There are a plenty of shops at Nai Sarak.
Bina Enterprises, AJ Atelier Architects
Chandni chowk has so much stores for architectual instruments