If you are keeping tadpoles in a tank in your home, they will usually nibble at the algae that grows on rocks in your tank so make sure you have a lot of these because they also like to hide underneath them. If you're a bit worried that there is not enough natural food growing in your tank, then tadpoles will also eat boiled lettuce, or if you have a bottom feeder fish, they could both eat alge pellets. Depending on how many you have, boil enough leaves for 10 to 15 minutes and remember to chop them into tiny pieces so they can eat them. It is essential that you don't overfeed them.
As they get older they will happily feed on crushed fish food flakes or even a few pieces of shredded mince meat, remember to keep their portions tiny so they can eat them. If you feed them mince, only put it in the tank a bit at a time or every so often, otherwise the tank water will go bad. You could even try finding them worms, if you can get any from your garden.
Another food that tadpoles appear to like are bloodworms. These can easily be bought from pet stores but as mentioned before, you can just dig up some worms in your garden and feed them to the tadpoles. Just remember that algae is their main food source when they are young, so make sure they have lots of rocks for it to grow on. When they're older, lettuce, spinach and cabbage leaves are also great.