Qui-Gon Jin was Obi-Wan Kenobi’s Master and killed by Darth Maul in the Episode I of the Star Wars movies. His costume is basically the Jedi traditional costume, but what makes the real difference for the Qui Gon Jin’s look is his the haircut, so you may want a long hair wig.
Anyway, I found one Qui Gon Jin costume for kids at Frank Bee Costumes, there is also supposed to be one for adults but it’s out of stock. And there’s a wig at Spookshop that should do it :
Check the wig at Spookshop.