Someone just linked this today:
Dog Clothing - Pet Supplies - Toys "R" Us
You might want to check out some of the for sale forums, here's one thread with costumes.
http://www.yorkietalk.com/forums/yor...e-10-each.html
Here's Yorkietalk Classified forum:
Yorkie Products For Sale - YorkieTalk.com Forums - Yorkshire Terrier Community
Target, and Walmart both have online stores, as well as Petsmart.