Looks like each class has at least a couple (to cover the different level ranges, 10-20, 30-40 and 40-50)
This TORhead search will show all the mod'able orange belts:
http://www.torhead.com/items/catg/2/subcatg/3/augments/eq/1/and/mods/eq/1/and/name/belt/sort/name,-basequality
From what I've read the belt and bracers are all random world drops.
Given the level spread, they should be available on each set of worlds.
Champions have the highest % chance to drop orange gear, so that would be the target of choice, but Elite, etc also have a smaller % chance.