Because it has been asked, I shall answer.
The girls' birthday shirts, as seen here
can be found here at Little One Boutique. The tags on the inside of the shirts made it easy to figure out.
And the Cute as a cupcake shirts that the girls wore to their party, as seen here
can be found at The Children's Place. Well, I actually don't know if those shirts are still available for sale but that is where I purchased them.