Hello Thomas,
I think you should have time to order and plant zinnia seeds. I don't usually start planting zinnia seeds outdoors until "danger of frost is past". Which I take to be about mid April here in east central Kansas. I think my zone is about 5b.
Maybe your climate is a bit warmer. Although anything connected with weather is, by its very nature, uncertain. Here it is safe to continue planting zinnias through the month of June. I have on occasions even planted as late as early in July.
ZM
