How can I use textures that are in a custom map i found?

You can use gcfscape and extract the textures, but I recommend asking the creator of the custom map for the textures

I would definitely recommend, rather than extracting the textures from the map, try instead to contact the map creator/whoever created the textures if you plan to use them in a project you are going to publish publicly. This way you have permission to use them and your not going to get into any trouble for stealing anything.

But if you do want to extract some textures, maybe just for personal use, you can extract them by opening the bsp in GCFscape. Sometimes however you will need to uncompress the bsp first since you can't extract textures from a compressed bsp

It's also totally possible that the textures you want are already publicly available. Check our Downloads section and the links to our packs on the home page, as well as the Bulletcrops Project.

Tip: Using the cheat command "suefaceprop" will output the name of the texture you're aiming at in the console.