http://www.boxingdesigns.com/shop !!!
$25+ shipping is not an attractive price for a niche tshirt. $20 is a reasonable price + shipping. The price would rather have someone spend that type of money on a brand tshirt they can use for any ocassion as opposed to a niche one that you'll likely use at home or during fight night.
Plus, Do the shirts really look like that or just for display purposes? They have this tube kind of shape to them with tiny sleeves. These type of shirts look great on normal fit solid material shirts. The fabric/build on those looks soft.
The designs are tight though. I would have picked up one if they were $20 a piece.
as someone mentioned earlier, ebay is good for boxing t shiirts, I've seen WBC hoodies, Cottos Ecko shirts and some TMT stuff, can't go wrong with ebay :)
eBay is always a good bet. Here is a thread I found a while ago while searching for the same thing, a lot of good posts:
http://www.boxingscene.com/forums/showthread.php?t=612865
Most of the sites are a bit cheap in terms of design but you might find a few you like. Another method I use is typing "name of boxer shirt" on Google images and then go to the site for it. If you can't find it for sale in that page, save the image, do image search, upload photo and it will come up with all the sites that have it available.
Hope it helps!
goldenboy has a store you can google, Danny Garcia has Dsgofficial.com Sergio MArtinez and Rios has Shopvde.com Floyd has Themoneyteam.com and you can find a few other fighters websites and plenty on ebay lol