You'll have to know the map names very well.If you somehow did not download the map the server is on, you can download the bz2 here and extract it with 7zip to put it in your tf2 folder manually.
http://fastdl.skial.com/fastdl/maps/the_map_name.bsp.bz2
Example:
http://fastdl.skial.com/fastdl/maps/achievement_idle.bsp.bz2
oh man, it doesnt work with vsh urban works, or with vsh degroot keep 2020You'll have to know the map names very well.