There is not a wealth of information, to be honest, and what there is is usually later period than what Belegarth would portray historically.
Here is a site with depictions of people/groups being labeled as "gypsies starting in 1475:
http://larsdatter.com/romani.htmWhether or not they are Roma is debatable, and the clothing may not be accurate. Mostly women are depicted.
Most of the same art, but with a pattern for what seems to be a commonality: a chemise and drape:
http://buttery.org/marian/Gypsy_dress/drape_main.htmThen you run into the problem of where your character is from: A European Roma will probably wear clothing different from those groups in the Middle East. And there's different thoughts on if they had different clothing at all, or tried to blend in.
My advice is to figure out a region/time period first, develop a little bit of a persona, and look for garb that would fit that. Be very careful when looking things up, sources for Roma anything range from bigoted POS to merely wrong to excellent.
For myself: I play a Belegarth-adapted Domari from Turkey. My garb is based on Entari's and eventually salwar (my linen harems do for now), which are adapted with bit of belegarth's fantasy culture.