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How can i find an FTP username
you wont be able to get usernames without exploiting the server to retrieve them.
as mentioned try basic ones, admin, administrator, guest, i dont know do research and try to harvest as much information as possible about the users you are cracking.
very illegal by the way. hopefully this is "theoretical" :D
aka PROTECT YOURSELF HARDCORE
Code your own FTP auth bruteforcer and use that. Let it run from your zombie-hacked pc's and while it's running try to 1) root the server or 2) exploit the website on their ip (if they have a website).
If you apply this method you will end up 1) having FTP access AND/OR 2) having root access AND/OR 3) being able to upload a shell or XSS the sysop (grab some cookies, phish him, whatever) or grab SQL files.