You can find such a device at an electronics store such as Radio Shack. You may need to use a little immagination to find a way to attach it to her clothes so that she doesn't lose it.
I recommend that you still limit the distance that she can go. 5 acres is an awful lot of space in which to look for her if she does need you. Perhaps staking and stringing a rope to indicate boundaries. I would also walk the entire space with her before she goes exploring on her own so that you will know what's out there.
I wish that we could let all of our kids explore their world, even in the city, as we did when I was young.
Michelle's advice about always being able to hear her is good. Perhaps you could modify a baby moniter.