State Line Tack usually has one or two showmanship jackets/tops, but they're pricey. I think what they carry is Hobby Horse brand, you might try their website for a little more selection. Anyway, Hobby Horse is by definition pricey .
Most showmanship jackets outside of those two places that I have seen are custom/semi custom made. A quick Google search brought up several companies, but rather expensive for my blood, and custom. If price is a factor, honestly eBay and tack swaps are your friends. There are some real gems to be found. If price isn't a factor, a Google search should offer several options.
My western G&S jacket is actually a deep green English hunt coat with flashier "Western style" buttons added to it. A bit plain, perhaps, but serviceable. Then again, I'm not a bling-y gal.
A stubborn horse walks behind you, an impatient one in front of you, but a noble companion walks beside you ~ Unknown