I found only one place in the US that sales them, in CA, they seem to sell plants at about $6.75 or so apiece rather than seeds but i suppose its better than having no source - and to the best of my knowledge they do ship so you can get some if your on the other side of the country...
i have never ordered from them or heard of them so im not recommending them (neither am i saying to avoid them, they are more than likely a good, trustworthy company) but i did find them via my secret little search engine, here is the link to their home page, i found it by clicking the K-M link to the right, Ctrl+F, and then typing the scientific name in to the very small bar that appears either at the bottom of the page or near the top after pressing Ctrl+F (which is the find function and will find a word or phrase on the page for you if it exists)
http://diggingdog.com/index.php
and oooooooo! they have perennial (instead of biennial) varieties of mullein, some of which grow to 8ft high in some climates!.... im off to go mosey around their site, i think i like! lol