These are completely free for you to have, just do a common name search... find the plant you want and if its available add to "germplasm" order. Once you get an entire list of as many as you want fill in the shipping info.
Remember, they want people who will 'TEST' these out for them for universities... the USDA themselves... and other companies. Most of the seeds or plants will come with a questionaire you can fill out AFTER they grow.
Also, don't forget... order extra for next year... just save them in a dry cool place and they should germinate well for you.