We actually have a list of bookstores in India that sell our books. You can find it here: http://www.manhattangmat.com/intl-book-stores.cfm#india
You'll see that on that list there's a bookshop listed in Kolkata:
UBS Publishers' Distributors Pvt Ltd
8/1-B, Chowringhee Lane
Kolkata - 700 016
Phone: 033-22521821, 22522910, 22529473
You can also try bookdepository.com or amazon.com, who ship internationally quite cheaply.
Student Services Associate