Here is a list of schematic sites you could download a schematic and upload to the server:
- Planet Minecraft
- MCBuild

Click here to download a .litematic to .schematic converter (jar) Credit:

CosmicX Schematics interface

To download a schematic, select it with worldedit on the server. Then do //copy and then //download. It will provide you a link to download your schematic on this page.