The primary source of OpenStreetMap data is https://planet.openstreetmap.org. This has the complete history of planetary-level data set snapshots, but these are extremely large files (50+ GB for the oldest ones). Note that YAAC cannot import OSM changeset delta files, only complete data files.
There is a web page of alternate sources on the OpenStreetMap wiki at https://wiki.openstreetmap.org/wiki/Planet.osm#Planet.osm_mirrors, for various subdivisions of the planetary data sets.
If you have Torrent software, you can also download the .torrent files
from the OpenStreetMap
website and use Torrent to download the actual whole-planet map data.
This will often be much faster.