Most of the GrowBarrel participants got these seeds from our workshops, but you can source your own too. We got our plants and seeds from Egg|Plant Urban Farm Store in St. Paul. A bit later in the season, seeding of spinach, radish and other herbs can be done at the end of August. Remember that handy grid you got at the workshop that has a schedule for transplanting, seeding and harvesting? Follow this link to see that page online.
This is another good reference from the University called Planting Vegetables Midsummer for Fall Harvest.