http://www.adollysworld.com/
I was looking for T shirts for Rebel (12 weeks, 20 oz) and have ordered several only to have them all be too big. Well, the website above is for doll clothes and teddy bear clothes, but the T shirts fit Rebel great! Once you open the website, click on teddy bear clothes and the smallest ones come up first. I ordered the American T for Rebel and it looks great on him and for only $1.75 each, you can't beat it!
A lady at the vet's office who has a TINY chiuaua told me about this place.