As far as I am aware, there is no legal download of Windows 2000 available from Microsoft, or anyone else for that matter.
If you still have the code of authenticity sticker on the computer, then you have a legal right to use the operating system on that computer. In the past, you could get a replacement disk, or restore disk from Gateway, but Windows 2000 may be too old now for them to offer such services.
As long as you have the legitimate license for the operating system, its my understanding that Microsoft doesn't really care where you get the install disk. But the members and staff at this site cannot advise you to look for unauthorized downloads of the operating system.