You can buy the seed from
https://www.cardnonativeplantnursery.com/ and maybe plugs if the time of the year is right.
Price sheet:
https://www.cardnonativeplantnursery.com/current-inventory-pricingLook on the first page of the price sheet (you will have to open the PDF). Look for any plant starting with the name Asclepias but if looking for only Swamp Milkweed, look for Asclepias incarnata
There may be other native seed nurseries around but this is the one that I use because it's the closest to me.