I order a lot of seeds from Swallowtail Gardens and have had a good experience.

I wish you had posted this last fall and I would have collected seeds from my many verbena plants to send to you! I didn't save any seeds, I just shook them off in my garden for more plants. The seeds are best sown in the fall here in my zone, then they emerge later than the mother plants and I find new sprouts all summer long. Verbena bonariensis is a perennial in my garden.

Cameron

Here is a link that might be useful: v. bonariensis seeds

Have you tried the exchange forum on GW? There may still be seeds available for trade or SASBE. It re-seeds in my area, but grows too tall if it's not pinched. It is quite beautiful in masses.