There are some super cute ones on Etsy. That would be my first choice. One of my favorite places I buy from is Bella's Prissy Puppie Boutique on FB, but there is a 6 week wait right now, so that would not work for you since her bday is next week. Or ask one of your favorite YT chicks if they can whip one up for you super quick. Check out the YT Vendor list too. This is one of my favorite dresses I've seen for bday so far, but it is a smidge over $40, but SOOO cute!
Dog CLOTHES- Dog Boutique-Custom made Designer Dog Clothing, Dresses, Shirts, & Holiday Apparel
Good luck!