Seems like you could make your own extract with a bunch
of knotweed and a bottle of cheap vodka (I like using 80-proof
vodka for this, because it absorbs both water-soluble and
alcohol-soluble compounds in the plant material).
Knotweed is an invasive weed lots of places, and ecosystem
management administrators would be only too happy for
people with some use for it to chop some down and haul
it off:
http://www.kingcounty.gov/environmen...-knotweed.aspx
I remember seeing it around local wetlands, the edges of
creeks, lakes, and ponds, and so on, although I did not know
what it was or that it was an invasive, non-native weed.