You should be able to get them in any store that sells musical instruments, almost certainly where they sell stringed instruments like the violin or guitar.
tuning forks are fine but they can be hard to hear.
What we use at home is an electronic keyboard. If you get one of these, make sure it's not an ultra cheap one where the pitch is not correct (toy keyboards often are like that). Brands like Casio make many keyboards and those should have no problem at all.
Or you could get the piano tuned.