I have used a company called Zazzle.com for awhile as an easy way to offer merchandise with my artwork on it. The website allows me to apply any image to a large variety of products including T shirts, sweatshirts, iphone and ipad cases, coffee mugs, shoes, cards, postcards, stickers, skateboards and yes, even doggie shirts. I like this setup because it saves me a ton of time and energy that would be spent getting these things made. Also they can be made one at a time, so I don't have to have any inventory at all, Zazzle handles everything but the design and I get a check once a month. I make very little from the products, a couple dollars each, but I love it because it gets my art out in the world in a practical way. The products are great too, I have sampled most of them. Very good stuff. You can even add text if you want, customize it. The only products I wont do are the prints they offer. The prints are good quality but I like to sign the ones I sell.
So if you would like any of these products with your dogs portrait on it please let me know. I would upload all 150 images but that would take days. Please email me (firstname.lastname@example.org) if you would like me to add your pup to a product. I just ordered an iphone case with my painting of Shaka on it, Ill post pics when it arrives.