How to extend our tourist visa?

No problem.

There are three immigration offices in Jakarta where you can renew your 30 day VOA for an additional 30 days.  A 15 day option is not available...the renewal is for 30 days only.   

Their addresses can be found here, and you can select the best location based on where you are staying in Jakarta.

http://www.imigrasi.go.id/index.php/en/ … on-offices

If you want to avoid potential long lines and the time you will need to take, there are tons of agents who can do this for you and on your behalf, but obviously costing you more than the US $25.00 to renew your VOA.

Either take the personal advice of some other poster here who may recommend one of those agents, or simply Google under Jakarta visa agents.

Go to any immigration office, many larger towns have one.
You can get a 30 day extension there at the same cost as your original visa.

http://www.imigrasi.go.id/index.php/en/ … on-offices