These days you can apply for and receive a 60 day tourist visa in your home country or in one of the countries close by to Indonesia and that 60 day visa can be extended at 30 days a time for 250,000 rupiah.
You will need a sponsor for this extension but seeing you will have been in Indonesia for almost 60 days by then you will have made numerous contacts - one of whom could be your sponsor.
Here is a recent post on LP Thorn Tree where timdog explains this very well.
Read Post 2. by timdog.
The number for the tourist visa is 211 which is the same as the SOSBUD visa you mentioned above which requires a letter from an Indonesian sponsor with a current ID.
This type of visa requires a lot of planning and so on.
What will you be doing during your 3 months in Indonesia - staying in one place or moving around the country side.
Also where will you be in Indonesia during this 3 months?