Source games have a wide variety of available maps. However, not all of them are best for custom game modes. For this reason, having a custom map is very useful.
Finding the Map
Since the Source community is huge, the majority of the best maps can be found in Steam Workshop
Installing the Maps
In order to install maps, you first need to create Steam Workshop Collection with the maps you'd like to use as guided here: How to create a collection?
Once you create a Steam Workshop collection, you'll need to give it about an hour for Steam to confirm it. Once Steam confirms your collection, you just need to insert your Workshop collection ID into Source Server Settings found on Game Panel:
Now head back to the Status page and click Update:
This will re-check the files for any updates and download the maps you've added to your collection.
And there you have it! you can now change the map either by logging in to the server and using the in-game console or by using the Game Panel console. Insert this command:
map <mapname>