If you don't know someone who will sponsor you, use an agent. Here's a previous thread regarding visa agents.
For the last six months we have been using BALI IDE, and to be honest it has been a terrible struggle each month to receive my passport back...It was never ready to be picked and most months they held on to passport ready for next extension, so in the six months I never had my passport, which...
Most agents will provide a sponsor letter so you can apply for a "sosial budaya" visa at the nearest consulate in your home country. When the visa is issued you will have 90 days to enter the country and the sosial budaya is valid for 60 days from your date of arrival in Bali.
About 2 weeks before the end of the 60 days, contact the agent again to apply for a 30 day extension. He can do this 4 x times (so the visa is valid for 180 days in total), then you must leave the country and start all over again.
In some consulates they insist on seeing a valid exit ticket within the 180 day period, but it's best to check with the consulate where you wish to apply.