It's too late in your zone. Miscanthus will have a hard enough time surviving a zone 5 Ohio winter anyway, but young seedlings will have no chance.

I have no experience with seedman, but they've been around for a long time.

Thank you.

I guess I could purchase seed now and cultivate them in pots over the winter.

That'll work, as long as you give the seedlings enough sun while indoors. I grew Miscanthus in zone 5 Ohio (Geauga County), and seldom lost a plant to winter. I was in the snow belt, though, so the crowns were generally protected by a thick insulating blanket of snow.