Regarding Humidifier, you have two choices.  Most of the BIPAP machines comes with Humidifier Attachment

See Below:

The other choice is to use a Room Humidifier. There are quite a lot of Room Humidifiers in the Market that one can buy.

In both cases please keep track of the Water Level and use filtered water.

For patients with Tracheotomy, there is a third option of using HME Filter (Heat and Moisture Exchange Filter)