Your answer for:
Where can I find maps?
Maps are automatically downloaded online
(unless the map-downloads server is down)
If you login and join any server a map on there will be downloaded for you.
Manual downloaded maps should be placed into /Beyond-All-Reason/data/maps/
Regarding custom maps
Out servers have a selection of the best maps, since we favor quality over quantity.
Also, good map quality has gotten a lot more important since we started using our new lighting/rendering techniques (like PBR).
Our maplist is Work in Progress (WIP), so if you have or found a really good map, please share it with us so we can look at it and possibly add it to the maplist.