Maps for 0.8.7 need to be either be downloaded externally, or copied over from existing games (CS:S, TF2, HL2 for 0.8.7, not CS:GO).
Maps for various gamemodes can be found here:
Note that these aren't maintained by Momentum people, rather by helpful community members. To install you'll need to put the BSP in
momentum/maps and ZON in
momentum/zones. Note that many of these maps are compressed - if the file is in
.bsp.bz2 format you'll need to extract them with a tool like 7zip. You can then launch the map with the
map command in console.
For any further questions, check out this post first which answers other common questions, or feel free to ask in our Discord. Hope that helps!