Well you can go here to the fs-scp website, and on the menu over there click downloads, and the rest should be self explanatory.
fs-scp websiteBut, while it is easy to get a hold of the older builds, i wouldn't know if the older mvp's are available or not still. And if they are, then that's a lot of bandwidth and waiting for a lot of mvp's for different versions to download.
For nostalgia. I started up with version 3.6.5, and it was nice, you could definitely tell it was a work in progress. Then 3.6.6 was about the same. 3.6.7 came along and that was when the mvp's really started taking off as well as the scp builds integrating new stuff like voice recognition. Although as nice as 3.6.7 was, it wasn't out for very long until the 3.6.8 mvp's came out as well as new cvs builds for them (at the time everybody was told to use cvs since there wasn't a stable official version...the cvs builds are still very nice today). And now we are waiting for the 3.6.9 mvp's, and that's going to blow us all away, nobody knows when (at least we have the 3.6.9 official build).
The other thing is that among experiencing bugs, you're likely to not notice anything different unless you have the mvp's for the corresponding version of the scp build they go for, and that's where you're nostalgia will be heavily nested besides seeing when new lighting affects were added, and the ability to play with ****ty directx (idk why some people just need to play in dx mode, ogl looks better and is faster among many other things).
While you're at you're project, you should take a good deal of screenshots, and start up a new thread something like scp history or something. I'd really like to see what you come across and see as i'm sure many others here would like to see as well (and because i haven't seen screenshots for anything older than 3.6.5).