For smaller pups, if you go to a Michael's (or other craft store), they carry stuff for bears and dolls, including plain T-shirts that you could embellesh. They aren't expensive - like $2.99!!
Here are my and my girls in our I
YT wear. Des's hoodies are made of a lightweight T-shirt material. Sissy is on the left in the photo. I'm not sure if I'm bringing them both or not.