Most of the major seed vendors sell at least a small selection of OG seed. Park's, for example, carries a Miscanthus, a Cortaderia, a Carex, a Festuca glauca, a Nasella, and Bunny Tails.

OutsidePride carries a couple of dozen ornamental grass and prairie grass seed varieties.

In my opinion, the best, by far, is Jelitto has the best assortment, listing 164 different offerings. They are in Germany, so shipping is slightly more than others, but their seeds are fabulous. While you're there, order a catalog, and look at their perennials as well.

You'll also find a lot of grass seeds offered in the GW exchanges. Be careful, however, of seeds that will not germinate, like unripe seed, seeds from sterile grass cultivars, etc..

When you look through the Jelitto list, you'll see that only a few of their grasses are 'named' cultivars. Most are simply species, and not the special named things that must be propagated by division.