I am installing an Aprilaire Humidifier 500a on an Armstrong Air Ultra V Tech 80 furnace. Where do I get the 24 VAC from the heating side of the furnace to feed the humidifier control?

It wants Common and W from the furnace (this detects heat mode)

And a separate, independent, customer provided, 24 VAC transformer with a minimum of 12 VA. Standard transformers are about 40 VA. Plug in transformers are available as well as ones that mount on top of a 4" x 4" junction box.