You could ask the server admins.
Many times they will zip the map and send it to you if you ask nicely and explain the purpose.
gaming.stackexchange.com/users/7803/tgrosinger
You can use World Downloader .
Press Esc in-game and press L to start downloading.
It will download everything you see — about 160 blocks away from you, from bedrock to the sky limit.
To stop the download, press L again.
Then, just look in your single-player maps.
gaming.stackexchange.com/users/18780/cooldudsk
It's theoretically possible.
Your client will download part of the map before rendering it, so the data is actually transmitted through network and stored in memory, and is accessible by either packet capturing or client modding.
Either way is straightforward to implement, and I implemented the latter one with MCP before for a certain complex labyrinth map .
However, such tools for retrieving server map can be easily used for hacking and unfair gameplay.
So don't expect any legitimately published ones.
Edit : I came across an implementation called WorldDownloader which may be good for your need.
You can try it out.
gaming.stackexchange.com/users/9437/xfs
As ursa_arcadius suggested, assuming you do not have direct access to the server itself (op/admin privileges in-game don't count), you could ask the server admin to send you the map file.
If they don't want to send you the whole map, but are willing to devote the time to it, they could take a copy of the map, use MCedit or a similar tool to export just your redstone circuit, and send you that export (which you could in turn import into your single player map).
See MCedit and how to use it for more info :)
If you do have direct access to the server itself, of course, you could just shut down the server for a few seconds (make sure no one is logged in!) and copy the map data directly from the server into your own MC world folder, and start the server back up.
gaming.stackexchange.com/users/9423/Doktor J
There isn't unfortunately, the only way to do this is through accessing the machine locally, or having some sort of remote access to the folder.
See my previous answer below for instructions on how to do this: You need to first locate the directory that the minecraft_server.jar is stored in.
Once you have this, there will be a folder inside here that contains the save data.
The default is world , but it may that there are lots of folders and it is hard to find -
if so, you can check the level-name property inside the server-properties file, which will give you the correct folder name to look for.
This also works the other way around, but you will need to rename the folder / level-name property to ensure that the correct world is referenced.
See also: http://www.minecraftwiki.net/wiki/Server.properties#level-name
gaming.stackexchange.com/users/8141/soulBit
As far as I can tell , this isn't possible.
You'll need to get your friend to zip up his save file and send it to you via other means.
gaming.stackexchange.com/users/3610/fredley
Depending on what you are using, (AKA: Remote host, Or Hosting on your computer)
If on an FTP Enabled server, then Copy the FTB Link from your Server Panel, And then Paste it into the Directory bar in windows explorer, then Copy the folder named "World"(Without quotes)to your desktop,
gaming.stackexchange.com/users/101977/Mods_o_joy
Is it possible to somehow get the map from an SMP server as a mere player? I did some large Redstone circuitry, and I would like to copy it into my SSP map.  If not, can I extract the loaded chunks from memory?
Can I copy part of a Minecraft multi-player (SMP) map to a single-player map?
gaming.stackexchange.com
<minecraft-java-edition>
gaming.stackexchange.com/questions/18190
gaming.stackexchange.com/users/7360/fjdumont
Try using World Downloader. You could also ask the server admin to send you the map file or try to access the machine locally. In order to duplicate files, the user has to  use either an external software or be in direct control of the original files.
To copy any file, it is suggested to use a third party software. It is implied that in order to copy files, the user need to have direct access.