The first time I tried to get duckweed for my tank (for the exact same
reason as you but with goldfish instead of an oscar) I was told by my
LFS that it was illegal to sell it here (in VA). However many places
will throw it in with other plant purchases if you ask them too -
almost all LFS workers say they hate the stuff and are glad to give it
away.

Incidentally I never did buy any of it - I went out to the great dismal
swamp (about 15 minutes from my house) and found a ton of it along with
hornwort, anacharis, and frogbit - all for free.

-Daniel