I would be careful about buying rain boots online we tried a pair on DS and even though he's been walking for 2 months he couldn't walk in these at all. He looked like a spaceman taking these very awkward steps!
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I'm not sure if they have them that small, but Joules Clothing (based in the UK, but there are a few places in the US that carry the line) has super cute Wellies.
Have you tried Lands End. I bought my DD an adorable pair there last year but I can't recall what the smallest size is that they carry. Her's are a size 5.
Re: Looking for rain boots..
These are size 4/5...
https://yellow-turtle.com/store/index.html?action=viewproduct&ProductID=116898286016586330&CategoryNum=114072815949064905