Where can I find arena, industrial, slow, needlestone, placard, wormy etc.?
The XML files are here: https://github.com/battlecode/battlecode-server/tree/master/src/main/battlecode/world/resources
If you install Battlecode version 0.2.2, the maps should also be usable from there.
Indeed they are. I did not find them in my maps folder, but they are contained within the JARs, which archon does not look in, it seems. Silly me.
You can use the archon copy-maps command to copy the latest maps to the maps/ folder! (assuming you are running a recent copy of archon)
Note that archon doesn't work on boxy_armageddon so you'll have to delete that.