Hi Ben,
"Where can I purchase seeds of old fashion tall (2-3 feet) single zinnias for my pollinator garden?"
I don't know of a seed source of old fashioned tall single zinnias, However there are some zinnias that have a lot of singles in the mix. One of them is Park's Whirligigs. Their strain contains a relatively high percentage of singles. And yet they are nice looking zinnias because of their unusual color combinations.
Another rather odd thing you could do would be to purchase some seeds of Zowie Yellow Flame zinnias and grow them this next year. They are an F1 hybrid, so their seeds are relatively expensive, but they produce lots of pollen, so they are great for butterflies.
And, then save some seeds from those Zowies next Fall and those won't cost you anything and, although many of them won't be exactly like the Zowies, many of them will be zinnias that the butterflies will love. And, then, save seeds from the best of those for butterflies, and you will be well on your way to breeding your very own strain of butterfly-friendly zinnias.
I breed zinnias as a hobby and, although I am not breeding any single zinnias, many of my zinnias have the pollen florets that butterflies (and hummingbirds and many species of bees) relish as nectar sources, and my zinnia garden had lots of butterflies this year. Including, somewhat to my surprise, a lot of Monarchs this year.
Last year I had a lot of butterflies, but very few Monarchs, and I was worried for them. This year many more Monarchs showed up, although I think they were just migrating through. But my zinnia garden served as a good refueling stop for them.
ZM