Volunteering in India - bookkeeping or accountancy for some charity?

For any work, paid or unpaid you will need a work visa. A quick search on the web and I am sure you could find plenty of charities and NGOs, just contact them directly with your cv, however the savings they would make by employing you (unpaid) will be minimal and may not e worth their while to obtain a work visa/permit, but you never know until you try. Good luck