I personally find it is best to buy clothes in store as it is easier to fit them right then and there. Its very hard to match sizes to our tiny dogs. My dog hates clothes and hates even more clothes that are even a little too tight.
There is a dog boutique where I live that cater for small dogs! They are online too www.palaceofpooch.com - they are really lovely in there and I can't recommend them highly enough!
I get most of Ote's from Elaina on this site or from Facebook groups and Etsy shops. I like the look and quality of handmade items, and my favorite "retail" brand is LuluPink!!