First option: the outlet stores. There are two in Mall of Indonesia and I've seen quite they stock large sizes. I suppose my asian frame fits their size perfect even when I was my old self and weight. Raja is the name and I read on their list of their other outlets that they have another one somewhere in Jakarta but forgot where. Your other option would be Bogor. They have plenty of outlets along the main street and I believe they have an outlet @ Jagorawi as well.
Option two: local tailor. I believe you can search in this forum to see of which members have gone to local their favorite local tailors who is willing to make you a custom fit clothing for a price but I'm not quite sure at what premium they charge you for or is it going to be a bargain or not.
I hope that helps.