Changing our old humidifier filter

It's very cold here today,we are in -4 degree celcius.We put on almost all our heaters coz we really can not endure the coldness even inside the house.My husband found out that our humidifier filter is kind'a old so we have to replace it.My husband was already looking for some humidifier filters on the net since we can get many choices and can compare the prices in different stores.We really have to change our old humidifier so that we can still inhale quality air inside our house and also to save some from our heating bills.