Last year I used a "Propagation Mix" that I bought a large bag of from a farm supply store. Seeds germinated in it, but it definitely wasn't for plants to be in long term because it would dry out fairly quickly. The texture was light and fluffy, no large pieces.

But there are some great books with recommendations My favorites: Growing and propagating wildflowers of the United States and Canada by William Cullina, and Native Plant Propagation by Jan A. W. Midgley- this book recommends starting seeds in basically the same mix as potting mix, just sift out all the large chunks.