First in a series about ways to find imagery.
Old Maps Online is not so much a source, as a map enabled directory for maps. You go there, find the place you want using the map, and look at the panel on the right side of the browser for old maps.
None of these maps are hosted on Old Maps Online. They are in various repositories world wide. This makes it important to check copyright and licenses for the sites before using the imagery.
Some may have URLs that are usable in JOSM or iD already. Others simply have a way to retreive the map. In the later case you can use [https://mapwarper.net] - see my diary entry with links to a number of youtube videos on Map Warper.