If you're unable to join a server because of a "missing map error" (see image 1), this tutorial should see you through. This is my first tutorial, so be nice!
Try rejoining the server to see if you're able to continue (or start) downloading the map. If not, proceed to the next step.
Open the "Create a server" menu on the CS:S main menu. Check your map list and see if the map causing the error is there. If it is, try creating your LAN server. If it works, you may be experiencing a consistency error with the server. Either way, proceed to step 3, and we'll try fixing the error by uninstalling, reinstalling or simply installing the map.
Using FPSBanana or the web, try to find the map you're unable to load. Download the map. Open the file the map came in, and copy the .bsp and/or .bz2 files.
Go to your main disk (C:, D:, E:, whatever) and open it. Go to Program Files > Steam > steamapps > (your steam name) > counter-strike source > cstrike.
See images 2, 3 and 4.
Right click > Paste into your maps folder. You can also drag and drop it into the folder.
You're done. Go play!
Before searching the web for the map you're missing, simply delete it from your map directory and check if you're able to download it through the server.
Somtimes you will also fall over this problem in a lan server and even if you do not even have the map or have deleted it, it still won't go any further.
I just recently found the way to fix it.
The way you should do, if you not already have read it in the console is to type in "net_maxfilesize #", the number should be 16 as default you will have to make it greater, like 120 or so, the number indicates how big the filesize others can download from you can be. as default 16 mb.
you can include what i've wrote in this tutorial because it is not the same as on a online server.