I had three pet ducks when I was growing up. We couldn't buy duck food here, either.

There are usually two kinds of chicken feed sold: starter food, which is for young chicks, and contains antibiotics; and layer food, which is for hens laying eggs for human consumption, and does not contain antibiotics. Just ask the salespeople at the feed store, and be sure to get layer mash without antibiotics. This is fine food for a duck.

Also, ducks like cracked corn, which you can also buy at the feed store. Soaked dog food is good for them, as are most vegetables. Mine loved watermelon, corn on the cob, etc. Just don't give them too much iceberg lettuce, it is useless. We used to go to the grocery stores, and get any vegetables and fruit that were going to be thrown away. Most of it was just fine, just a little wilted. Our ducks got a huge variety of food this way, for free!

You can also supplement their diet with live feeder fish, if it doesn't bother you. I personally couldn't do it, but it is really good for them if you can. You can also give them tuna or salmon, as long as it is packed in water and rinsed well, to get rid of the excess salt. Sprinkle some pond koi pellets on their food, too.