Towards the end of the summer, into fall is the best time to score seeds from plants you see around you, David. Most gardeners "deadhead" their zinnias to keep them blooming as long as possible.
As with many flowering plants, if you let them make seeds too early in their flowering season, they will end up putting all the plant's energy into making seeds and will stop flowering.
If you befriend someone with a garden full of nice zinnias, maybe they will save the flower heads for you. The very best way to score free seeds of many colors!
