In our house we have the Airwick brand. Those little buggers can be expensive. Even with coupons they can still be a couple bucks. So instead here it what I do.
When the original scent is all used up. Take the empty glass refill.
Pop off the plastic insert and take the smelly stick out too. Rinse the glass container in warm water.
Take your favorite essential oil. Add 10 drops in the bottom. Fill about 3/4 of the way up with water. Put the plastic insert back in and you now how a much less expensive version of a scented oil. Pop the glass refill into the electrical case and plug it back in.
Enjoy your fresh smelling home!