So I had a cracky headcanon that Jehan wore chain mail to the barricade, but then wondered if he could actually have had access to at least a mail shirt. Which leads to this. Yep.
Going with a personal headcanon that the Changer is part of a ring of clothing-suppliers, and that one of said suppliers has supplied Les Amis before, he could have just bought a used one, such as this.I want one.
But using a source that's not canon seems like an easy way out, even if that does seem like something he'd have and have access to. Even assuming that he would associate with the Changer himself seems too easy; it's assuming that they would talk to each other along with assuming that the man somehow has access to armor from India/Asia.
So let's look at Jehan himself. He's the only son of well-off parents, so there's money for these endeavors.
If Hugo had only made his barricade literally in 1848, Prouvaire would have had possible access to a "bulletproof vest" of mail from Ireland. But Hugo puts us in 1832. So I guess we're going back to a blacksmith or Jehan making it himself
Supposedly, so long as you have the time and materials, chain mail isn't actually that hard to make. This is a decent tutorial.
There's no reason a blacksmith wouldn't be able to make the rings at least, I wouldn't think, and the electric drill bit used here could probably be replaced with a crank or just turning the rod by hand if Jehan decided to make it himself. I have no clue as to what the price would be, though, or whether or not a smith would teach him how to make the thing if he wanted the Do It Yourself Route (Google Books is empty of 19th-century How To's for this).
Of course, he could always just buy a shirt like at the beginning of this post, but then the question is who would he get it from. He could have other friends with the right connection, I guess, but that doesn't seem like a really solid answer.