You can find servers on server lists. If you're looking to build freely using WorldEdit on a public server's world, you've got slim chances of that occurring. Only special staff members are able to use it, due to not only its potential destructive power, but the fact that you can significantly shape the server by building large things in it; public warps should only be built if it's good/suits the server and the owner approves.
If you're looking to become a staff member on a server just for this purpose you might end up losing it... or gaining it on a server with an unwise owner who promotes people very quickly, even if there's already too many or they aren't needed. You can also install the singleplayer commands mod, or run a local host server on the computer you're on right now and install plugins you want.