I love growing amaranth! I store it as a grain and cook with it frequently as I would quinoa and also as supplemental feed for the birds in the winter. I get my seeds from Baker's Creek. You'll only have t do it once, as it puts out tremendous seed heads. Here's the link. http://www.rareseeds.com/search/?keyword=amarant
So far I have tried five varieties. My least favorite is the Golden. While beautiful, the leaves were more on the bitter side and the seed heads are harder to tell when to harvest.
This is the first time for me, but not the last! Interesting that you use it as quinoa. I knew about the young leaves tasting like spinach, but knowing that the seed can be eaten makes it even more appealing. I imagine the birds will let me know when its harvest time!