All a humidifier does is increase the relative humidity of the room. Couldn't you simply mist his enclosure until you are able to replace it?

If it is actually the beginnings of an infection, a visit to a qualified reptile vet is your best bet (as you already know). Treatment for respiratory infections vary depending on the actual organism involved, so it's not something you want to go about trying to find a home remedy for. You could end up chasing your tail and wasting perfectly good time and money on something that was never going to work in the first place.

Good luck to you and yours, and welcome to the site!

Best regards,
Eric