There are a few options. First, just directly inquire at the Dell Support No. 000-800-440 2475 if the battery is available anywhere in India and if so how to place an order for it and have it delivered to you. It is possible that since the product is from the USA, the hardware parts, including the battery, are not available in India.
Secondly, you can check for yourself at the official Dell India website if they have a battery for the particular model that you have, which is - https://pilot.search.dell.com/6554/laptop%20battery. Alternatively, go to this page - https://www.dell.com/en-in/shop/partsbytype/batteryselector. They have provided the list of the type, such as Vostro, Inspiron, etc and then a list of all the model numbers. At the same page, at the side, there is a support Chat Tab that you can open. It gives the contact numbers as 1800-425-4002/080-2510-8001 and an email support ID is also provided there.
Thirdly, you can seek technical support through this page - https://www.dell.com/support/incidents-online/en-in/contactus/Dynamic?spestate.
As for using the power bank to charge the Dell laptop, I would not advise it unless you know for sure that it is compatible with the particular model. I am not knowledgeable about electronic gadgets, but I assume that just as it is advised to have a compatible charger for a smartphone, so also it may be the case with the laptop. I think it has something to do with the voltage or some such thing that may cause electrical fluctuations and perhaps not be safe. So you can inquire with the Dell Support people which power bank is suitable for the laptop that you have and if the one that you have is not so, then whether they have one available with them and place an order for it.
Note that it is always best to contact the official company of any electronic product to get authentic, original hardware to replace the earlier one. In case you consider purchasing it from some e-commerce site, there is the possibility of getting a second-hand used version of it that will wear out soon and, contrary to the claim made that it is compatible with the product you have, it may not be so at all.
When you make a commitment, you create hope. When you keep a commitment you create trust! ~ John C. Maxwell