Where you are, you should be able to grow A. tuberosa, A. incarnata, A. syriaca. When I lived in OH, I found plants at stores occasionally but haven't seen any since moving to AL. AFAIK, all are easy to grow from seeds found at the right time, if you like to do that kind of thing. Many people save them to share for the cost of postage, you may want to investigate that.
The blooms aren't important as far as being a host plant. It sounds like you have other nectar plants to satisfy that requirement, so even if your Asclepias didn't bloom the first year, caterpillars could still munch on it. Depending on how many caterpillars you get, one or two big host plants in the midst of many other nectar-producing plants should be sufficient. I don't know how big of a garden you're talking about, but the ratio of one host plant to 15-20 nectar plants (depending on size,) should be a good basic guide. You'll get a feel for it as you go along.
Some annuals (where you are) that make great nectar plants... Coleus, Basil, Pentas, Zinnias, Lantana (some might be hardy where you are.)
