Butterfly Milkweed is not on the PA invasive plant list. The following link will provide additional information on other PA invasive plants, shrubs, and trees. http://www.dcnr.state.pa.us/cs/groups/public/documents/document/dcnr_20026634.pdf
I can't tell you where you might find milkweed seeds locally (check with your local commercial nursery) but you can certainly order them online from most nurseries. The seeds are very easy to grow and do well when planted in the fall or when cold-treated for three months prior to planting. Butterfly Milkweed will spread through seed distribution and underground shoots.
Other plants you might consider to attract butterflys are pentas (annual) and coneflowers (perennial) both of which you can find at any of your local nurseries or big box store garden departments.
Good luck in attracting the butterflies.