I do this all the time. Cuter and cheaper than dog clothes.
For the girls, I look for little tops or dresses that button in the back. I cut up the side seams and across the back to remove under the belly, turning under and hemming the raw edges, then I move the buttons over to make it fit under them better. I add a little velcro if needed. Here is a couple pictures of the front and back of some I altered for my girls.