If you can't find any, perhaps you could have osme custom-made.
Marnie at One Wet Pants does made-to-measure knickers, mostly aimed at babies/young toddlers doing EC, but I'm sure she could do a size 2-3 ... maybe you could send fabric to her (bamboo jersey or org. cotton jersey)? Worth asking ...
Noonee Wilga - Home - custom-made training pants is her website.
Oh yeh, and washing powder was the first thing that came to mind for me - even if she doesn't react to it on her other clothes she might just be more sensitive in the undies area. And even if you used the same powder/liquid for nappies, you'd probably have been using less of it and/or rinsing more.