Buying duck and lemongrass in PVD area
For lemongrass, there are numerous asian groceries that will serve your needs. two that I frequent are Five Star Market on Dexter St south of Cranston St and New Asian Market on Broad St near downtown, next to the McDonalds. I prefer Five Star, but both will have lemongrass.
I bet both will have duck as well. Otherwise, I'm pretty sure that Antonelli's on Depasquale Square has duck.
(Just remember to get it a day before you want it--the birds are slaughtered to order and in my experience you do not want to cook it a couple hours after slaughter, as it will be very tough--due to rigor mortis. takes a little while for it to tenderize after that. i don't know the details, but i did make one inedibly tough dish from there.)