How do I wire a humidifier without a 24 VAC IN terminal?
Your furnaces controller is typically powered from a 24VAC transformer.  Trace the incoming line-voltage power source - it should connect to a transformer. You can verify the output voltage is 24VAC with a multi-meter.  Even if there is not a dedicated terminal, you can connect to the transformer output in parallel in order to power your humidifier.  Often there will be a wirenut already present, but if not you can cut the transformer output leads, strip both ends and as well your output to your humidifier and connect all the stripped ends with a wirenut or other sutiable connector.

Alternatively you can power it with a dedicated 24VAC transformer.