generally, seeds packaged for the growing year can be planted in spring or fall depending on your region....
so you can go to your local garden center, or some of the online seed companies...
i have had some luck here (but, keep looking if you want certified "organic", these guys just sell seeds, all kinds of seeds, at a reasonable price)
http://www.willhiteseed.com/ps:
i had great luck overwintering some of my salad greens (lettuce, chard, spinach mustard). even had fava beans, mustard and swiss chard surviving the Feb2010 freeze here in Dallas...i had written them off, and stopped covering them...2 weeks later i was eating salads from the garden (go figure).