Depends where you look.

Here's some on the Missouri ...

... and here in Thailand ...

There's a lot of it about.

Maybe you can drip vinegar onto it,,,,, and see if it goes 'fzzzzz' with some bubbles of gas appearing.

Limestone is the result of trillions of small crustaceans dying and turning into sediment at the bottom of the ocean of the time.

Look for old ocean deposits.

Would need to know the location you live in. Otherwise, contact the local university and ask the geology faculty where to get some.

Ever pick up a stone and draw on the local street or other rocks as a kid. You most likely used limestone to do this.

I may be wrong but in my experience look for grey rocks that usually have a grey white powder on them. They should be able to leave a "chalk" mark on other stones without too much effort. Road bed stones are more often than not Limestone.