Google highcountrygardens, one word, and request their catalog.

Others will suggest other good catalogs.

Gardenwebs ornamental grass forum has some participants that are good at answering question.

The best online retail source for ornamental grass seed, by far, is Jelitto. They are a very reputable German seed vendor, and ship US orders from Louisville, KY.

Here is a link that might be useful: Jelitto

Germania Seed and Jelitto Seed would be my two recommendations for ornamental grass seeds.

HCG would be a good source for potted ornamental grasses.

You'll also find a large ornamental grass selection at any of these mail order suppliers:

Bluestem Nsy
Santa Rosa Gardens
Plant Delights

Remember that many of the popular varieties of ornamental grass are those that will not come true to form from seed, or the seed itself is not viable. These varieties must be produced by vegetative propagation, which means you'll need to start these from potted grasses or starter plants.

Being in zone 5, wanting to start grasses in mid-october, you'll face some challenges. It's too late to start from seed, unless you have a hot-house to grow in. It's also too late to plant out most mail-order grasses in zone 5. You may want to think about planning out for spring 2008 and starting as soon as warm weather sets back in.