Finding seeds of the different Impatiens can be next to impossible. They dont have a long shelf life. The big breeding companies dont produce seed for just this reason. I did get some of mine to set seed many of these were open pollinated this winter by Humming birds. So I let most of them pop all over my greenhouse. I think most will be true, hybridization does occur in Impatiens but is not so easy as other plant species. Once the Hummers found the Impatiens they liked them but I dont think they are first choice its just that there was nothing else blooming in the garden.
Plants are your best bet. You can find plants at Kartuz's greenhouse, Annie's Annuals, Heronswood and Plantdelights.

Thanks, Derek. the zombensis is doing fine for now. I have 3 seedlings at 4 true leaves. What are best growing conditions and what is cold hardiness? Tony at Plant Delights only carries omeiense (I got mine at forestfarm) and 'Sichuan Gold'. Annies Annuals seems to have good selection, so I'll try that. Are there species you recommend particularly? Is there a book?