You can usually get duck feed/duckling feed at any feed store, or also check local exotic pet stores if you live closer to or in town. (Pelleted)Duck feed looks similair to rabbit food, but instead of green the pellets are a light tanish brown. Duckling food looks very similair, only smashed into fine crumbly pieces.
If You can't find either of these, chick starter (which any feed store will have) or game bird starter will work fine. Make sure you get unmedicated food reguardless.
Where I got my duckling food, it costs 0.79 per pound. However, this is very expensive for duck food, as I bought it from an exotic pet store.
I now feed my duck duck/chicken scratch, which costs 5.00 for 10 pounds. However, you can't feed ducklings this scratch, as the cracked corn in it is too much for their digestive tracts. This is just a suggestion for when your duck grows up.
You only need to change their food once a day. However, if they spill it, you'll need to refill it, so make sure you get a sturdy bowl.
--Ariel