I bought one at a Civil War encampment. They sometimes have a suttler's tent.
But you can find clothes online. Use various combinations of terms in Google. (vintage, reenactor, 19th century, Civil War, clothing, etc.)
Someone will probably have better ideas but I'm certain you'll find something using those terms.
I can recommend Gentleman's Emporium (http://www.gentlemansemporium.com/store/vests.php) with the caveat that their stuff is not necessarily 100% historically correct for the period in terms of pattern, construction or materials - but their stuff is well made and their customer service is good.