If you want to have a server that plays something other than just the few basic czero maps, you're going to have to install those maps onto your server by hand. But don't worry: Installing new maps to your server is pretty easy.
1. Pick a site to get maps from. I have two sites I really like: cstrike-planet
and whatever-the-fuck-game-banana-is-today. I prefer cstrike-planet because it's waaaay less buggy than fpsbanana, and a much faster site. But I think fps banana actually has a bigger map database. If I'm looking for a specific map, I will usually check cstrike-planet, then fpsbanana, and if neither of those two sites have it, I will just use google.
2. Download all the files. They usually come in a zip folder or something. Most maps require more than just the .bsp file, and almost every zip file you download is structured with the correct directory structure.
3. Put the correct files in the correct directories. For example, put the .bsp file in the maps folder. Put anything in the "models" folder of the zip file into your "models" folder of your czero folder.
4. Put the map name in your mapcycle.txt file and/or your maps.ini file - whichever file you use as your map-changing file on the server. (Your maps.ini file is found in your addons/amxmodx/configs folder.) I always put the name in both files, just in case I decide one day that I don't want to use the maps.ini file anymore, at least I will still have all the maps listed in mapcycle.txt.
5. If you use the Change Level section of your amxmodmenu to try to change the level, the new maps won't show up in the list until the next map change, so don't get worried and think that you did it wrong. (AMX Mod X files only get reloaded every map change.)
6. If your server crashes when you try to change the map, it's probably because you're missing an integral file. verify that you put all of the downloaded files in all of the correct directories. It's pretty easy to accidentally drag the file into the wrong directory or something.
NOTE: A czero server can run 1.6 maps, but a 1.6 server can't use a map that ends with _cz - those are czero-specific maps. Neither server can run a CS:Source map.