As you've probably seen, you can download older versions from the EERE previous versions page. However this currently only goes back as far as version 7.0.

I don't know of a source for older versions than that. Someone on here may have it in the dark recesses of a hard drive somewhere. You might want to add what operating system you're using to the question.

Putting my oar in though, I'd recommend biting the bullet and moving to the current versions. Not only are there more features, there are also bug fixes and much improved documentation/error messages. You can also migrate your old models using the multiple version transition tool.