There is a website: www.friends2support.org where you can search for a particular blood group. There are more than 60 places from India which include almost all the major cities including metros, listed over website where volunteers can go for registering as Blood donors. Also there is information about 2000+ hospitals available over the website.
I you are looking for blood donors, you’ll get thousands of donor addresses. You can post request for blood by going to Blood Request page.
I have bookmarked the link!