hello there kenji and all. i'm not in jakarta but in bandung.
as redstarcap mentioned, you could visit the sepedaku.com site.
there are many people using road/mtb/folding in the city, but it seems that the fixed gear scene is more active on the city.
you could visit bikeshops like RodaLink, but there are many LBS too. not like any in japan though. but i think RodaLink will suffice.
if you need any assistance, i can get you in touch with the people there.
and i think uncle Tedi will help you too.

cheers.