From where do the bees get propolis?

The workers forage to collect plant resins, bring it back to the hive and mix it with wax and possibly with their saliva. Conifers, maples, the poplar family, including our native cottonwoods, and other resinous plants., provide ample sources in Western Washington.

Picture of propolis with uses inside the beehive taken from a review article in the journal Insects, "Propolis Counteracts Some Threats to Honey Bee Health" Michael Simone-Finstromet. al.