What is the botanical name that you have for spiderflower? For purple coneflower, I would plant them now.

You could try the North American Native Plant Society NANPS
(nanps.org?). They are reasonable, and will allow non members to buy once the members get theirs. Try the site and see.

Try the exchange forum.

http://forums.gardenweb.com/forums/exnative/

Although NANPS will likely have seed, they are based in Toronto and the US gov't has made it very difficult to ship seed into the country.

Native plant seed sellers are available on the web, try Wildflower Farm or Prairie Moon Nursery.

There are several native purple coneflowers, I assume you mean Echinacea purpurea, don't buy a hybrid or a named variety if you want the species. There are also many spiderworts, you will need to specify which one when buying seed.

If you buy the seeds, they will come with sowing instructions. Most native prairie plants require cold moist stratification before germination will occur. That means that the seeds need to be planted in moist soil or potting soil and then keep cold but not frozen for several months.

The easiest way is achieve cold, moist stratification to sow the seeds in the fall and let Mother Nature do the work. If you sow in the spring, the seeds will likely stay dormant all summer, undergo cold moist stratification during the winter and not germinate until the next spring.

Here is a link that might be useful: Prairie Moon Nursery Link