The Florida nurseries have it all the time listed as octopus plant it may not be japonica but is pretty close to it and is none the less a great aquascaping plant I think the name is I. vellata
I will soon post pictures of a layout with a nice section of Isoetes.
AZGardens carries a variety of Isoetes (I. lacustris I believe) that a friend of mine has in her tank. It looks very distinctive and unique among her other plants. Will be trying some myself as soon as she propagates it