For example, here are the maps of the 2A/KLM server:
https://www.mohaas.net/app/?q=html/mapdownload/100You can replace the number 100 with any other server that uses custom maps
and is registered with mohaas.net.
Download the maps, and place them into your folder mainta. To make sure, they
work correctly, you should rename them to z-****.pk3, for example mp_anzio_dm.pk3
to z-mp_anzio_dm.pk3 and so on.