My local garden stores carry seeds from Lake Valley Organics. Below is Lake Valley Organics Mizuna (Japanese Mustard). Planted in my Harvest on August 28. I was afraid that it would taste too "mustardy," but it makes for a nice, mild salad.
From Lake Valley's website, "Brassica rapa (Japonica group) Tasty, quick-growing green for mixed salads and light cooking. Best when grown during cool weather and harvested young. Very sweet flavor with just a hint of spice. Easy-to-grow “cut and come again” leaves."
Since it is a“cut and come again” variety, I gave it a short haircut and expect it to grow back soon.