As much as I hate recommending the USPS for anything, they actually provide an official search tool for this on their site:
https://www.usps.com/locator/ (collection boxes > Blue Boxes)
He never fails to brighten the day of whomever greets him at the door at our house :)
I suspect the Payphone Project's mailbox locator was one of the first crowdsourcing web applications and predated the USPS effort.