There are lots of midis out there, but knee length might be a bit trickier to find without doing a custom order. A lot of the Lithuanian linen shops on Etsy will do simple customizations like adjusting the length for little or no added charge, but overseas returns are complicated and expensive, so not the place to look if you want the option to return or exchange. That said, the quality is fantastic: thick, drapy linen in gorgeous colours. My favorite shop is Not Perfect Linen (example in Finds), but I've had good experiences with Linenfox and Magic Linen as well.
For US brands, in addition to Eileen Fisher and Flax, you could check Pyne & Smith. They are better known for their linen dresses, but do offer some skirts as well. Tradlands might be worth a look too.
Poetry, Garnet Hill, and J. Jill might have something?
