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)

The only thing I like about USPS is my mailguy, who is always smiling when he rings our doorbell on occasions when we have a shipment from Amazon and it's too big to fit in the mailbox.

He never fails to brighten the day of whomever greets him at the door at our house :)

You don't like cross country mail for half a dollar?

Interesting. Thanks.

I suspect the Payphone Project's mailbox locator was one of the first crowdsourcing web applications and predated the USPS effort.