You can also try www.zazzle.com They have just about everything and they have very personalized service so I'm sure if your friend emails them and asks thme about a dog shirt they will do it for her..that is, if they dont already have it on the site.
I found a couple patterns today to sew that are matching outfits
If she likes to sew tell her to look in the big pattern books they
have at Wal-mart I found them under the craft section (why in crafts I don't know) you would think they would have a seperate section for pets in the
pattern books by now.