I believe 10.7 and 10.8 cost money (unlike 10.9 and 10.10), so you probably wouldn't be able to download them legally for free (unless you had already previously purchased them via the App Store).

The only LEGAL place to get these installers is from Apple.

If you haven't already purchased Lion/Mountain Lion, or downloaded Mavericks, you may still be able to get it. Here's the Apple Store link to purchase Mountain Lion (they email you a redemption code, which you can enter into the App Store):

http://store.apple.com/us/product/D6377Z/A/os-x-mountain-lion

Here's Lion:

http://store.apple.com/us/product/D6106Z/A/os-x-lion

Not sure about Mavericks installers, but again, if you have ever purchased them, they should be in your Previous Purchases links.

http://osxdaily.com/2014/02/24/re-download-older-os-x-version-from-mac-app-store/

You may need to "un-hide" them:

http://support.apple.com/kb/HT4928

You can also get them if you have a mac developer account. I just looked and 10.7->10.10 are available for download