Try:

Santa Rosa Gardens
High Country Gardens
Plant Delights
Bluestone

if you are looking for liner trays, try Perennial Market.

A few sellers have the "Giant" on ebay if that is what you really want, just search for "Miscanthus Giganteus" on ebay. I see a couple of sellers that offer small pieces for like $15, not much risk. Seems like your best bet.

Don't think you need very big clumps to get started, it grows quickly. Its already growing in my yard almost 2 feet high, the sooner you get started in spring, the better.

As for other plants, Miscanthus sinensis "Strictus" would make a nice upright dense 6' high wall, the blades are rather stiff and vertical. "Strictus" is easier to find, and would make a more dense fence.

The very common "Gracillimus" can get 5-6' if well planted.