You can go about it in two ways; you can be put on a list of available respite providers, or you can be a respite provider for an individual(s) or a family.

The 1st way you are available to who needs you, but you can still specify what age groups you can handle. No everyone can handle babies or teenagers and vice versa. My wife and I foster drug addicted and exposed kids from 0-5.

The second is often done by family friends or even other family members that want to help someone they know specifically. Obviously rules are different everywhere, making doing the right thing that much harder.