apparently you can stick almost any piece in the ground and it will root in 2 to 3 weeks

do a bunch of cuttings early/mid fall, keep them under some fluorescent lights until spring....i personally havent done this with porterweed, but have with some other plants

Any big box, ie Lowes or Home Depot, should have it. Look for it in March or April.
I've got some around my mailbox, both red and purple, and the hummers really like it.