As you rightly said Aadhar Card is now mandatory for any kind of transaction whether it is online or offline. But for getting Aadhar card is a little bit tedious job as no government / private agency is offering to do the same in online. But for getting the details of the nearest enrollment centre, you can get the help of Aadhar official website "www.uidai.gov.in".
Once you get the place where from you get it enrolled, you have to visit the place in person in the early hours well before opening the office and get the token number(which is being issued in many centres) to be enrolled for that day. You have to carry ID Proof and Address proof alongwith you for enrolment. If you want to get the card for all the family members, all the members must be present for enrolment. This process is being done with free of cost since the fresh enrolment. Once the successful enrolment is done, you would get the card within two to three weeks by post.