|Asclepias syriaca colony|
As for more uncommon milkweeds, Asclepias viridis is probably the largest flowered milkweed in the US. It has a south/central distribution but is perfectly hardy here. It tends to recline as it grows and right now the magnificent flowers are at their best. A bit earlier is prime time for Asclepias sullivantii, the prairie milkweed. It can spread underground so it has to be watched, but the leaves have a colorful midrib and the flowers are a bit darker and prettier than the common milkweed. The plant also tends to be shorter, maybe three feet at most. Neither species is native to NY to my knowledge but they seem happy here, and they sailed right through our hard winter. I amended the heavy soil here with road sand so it drains better which probably suits them fine. I did have an Asclepias purpurea, a lovely species in one of my gardens without amended soil and it perished, so I will have to try it again in a better position.
Most asclepias are hard to find as plants because they often don't look their best in containers. Its the knowledgeable gardener that will know their potential beauty, and either buy plants if found or raise them from seed. Seeds are easy to grow, the most difficult thing is finding the odd species and also remembering that most do prefer a period of cold stratification before they germinate. Once they are growing get them in the ground as soon as practical for they flourish better in the ground than in pots.