While 10.5 is the current latest release, you should be able to download 10.1, 10.3 or 10.5 directly even when using the latest Installer.
For any older versiosn you should contact SAG as Tong suggested.
When starting a new project/system, I would suggest to go for the latest release directly.
#webMethods#webMethods-General#Integration-Server-and-ESB