the website "tribal baby" (an EC-ing site) has shop links, and they have places that sell tiny underwear.

noonee wilga (an australian WAHM) will make tiny knickers, you provide your child's measurements. she also makes training pants (called One Wet Pants) to measure. If you sew yourself, Noonee Wilga sells a pattern, to make them yourself.

size 2/3 seems to be the smallest size in most Australian shops (like bigw, target, kmart etc). i had the same drama with my 11kg child at two years old, so i went on the same hunt as you.

i actually met a baby who was 7 months old, and wearing knickers - and keeping them dry too - as her parents used the EC method. blew my mind that a tiny bub could and did wear knickers.

I chose not to EC, but found the tips i read on the tribal baby site, very useful to read, to help me toilet train my two year old.

hth