You can buy this tshirt at Red Bubble: http://www.redbubble.com/people/garykemble/t-shirts/6298753-1-guns-dont-kill-...
Or, given the design is under a Creative Commons licence (Attribution-Share Alike) you can make your own t-shirt (download the jpg from Flickr: http://www.flickr.com/photos/garykemble/5187025370/)
The image in this design is also under a CC licence. You can view the original here: http://www.flickr.com/photos/simonov/1971164711/