Oh I hated that so much when Poppy was younger! The really small sizes are so hard to find. I think I only had one item for her when she was that small LOL I found it on Ebay but I agree they dont have much choice.
I'm thinking maybe cat/kitten clothing? I know
www.petzcrazee.com sell clothing for cats as well as dogs. I have never looked at the cat clothes I have just seen them advertised on the site, but I buy a lot of Poppy's clothes from there and they are really cute.