I order Burpee's Burpeeana Giants zinnias from Burpee's. I order Whirligig zinnias from Stokes Seeds. I also don't hesitate to order zinnia seeds from Parks, Harris, Johnny's Selected Seeds, Thompson & Morgan, and Hazzards.

ZM

I only order from Geoseed.
Best prices and service. Their pricelist says they are for "professionals", but they have always been extremely polite and do my order regardless how many packages I order.
I just received their 2012-2013 pricelist. The one on their site is still the 2011. No pictures in the 100 page pricelist, just a phone list of hundreds on types of plant seeds. If you want an easier copy of their online catalog, click the online catalog and click the save icon at the bottom. You will download an easy to use PDF.
Only draw back is that I think they expect you to be an expert at the seeds you buy; no growing instructions. And if I mess up on the germination, well that's my fault. I don't good back for a refund like I use to do with some overpriced seed companies.