I'm looking for a public speaker on the topic of India. There are a
lot of poor people in India who are in desperate need of economic
development and international aid. The (unstated) goal of this event
would be to get the audience thinking about these issues and hopefully
convince them to give money. All I need is a speaker.

They need to be -
1) At least somewhat famous.
2) Interested in helping the poor in India.

Good candidates would include: respected current and former leaders
from India, activists and reformers from India, or even non-Indians
who have involved themselves in helping India.
This event would take place in the USA. 

Please suggest some possible candidates.

How about Deepak Chopra? I also know that the Bill & Melinda Gates
Foundation has done some work in India. They might have someone that
can speak on the topic or help identify ways to locate a qualified
person.
Good luck on this - being an Indian, I think this a much needed service.

I just remembered another, and I think she is exactly who you're
looking for, Arundhati Roy, author of 'The God of Small Things'. I saw
a documentary on PBS that featured her work on behalf of poor
villagers opposing the building of a dam.

Depends on the Target Audience. (and How much you can afford)

Jagdish Bhagwati & Amartya Sen are two leading economists that come to my Mind.

Then there is C K Prahalad, who believes the Poor are a Giant Business
opportunity (I agree) and not in need of "convention help" - aid, in
this case.
He is, I quote,
 Harvey C Fruehauf Professor of Business Administration, professor of
corporate strategy and international business at the University of
Michigan and an independent consultant.

Arundhati Roy would be OK if all you are gonna do is generate sympathy etc.,
afaik she is not held in high esteem in India, unlike the above, she
is perceived to have hijacked the anti-dam movement for her own
publicity.