Older versions of QGIS are only archived as Windows exe and source code. You can download older Windows versions from http://qgis.org/downloads/
To run them on a Mac, you need a Windows emulator, or a (free) virtual box which can contain a full version of Windows (supposed you have one, and register at Microsoft).
Alternatively, you could install a free Linux version inside a virtual box.
For Debian, you can have QGIS version 1.4, 1.7.4, 2.4.0 and 2.14.10, and for Ubuntu you can have QGIS 2.0.1, 2.8.6 and 2.14.5 depending on the Debian or Ubuntu OS version.
All these are free of charge, and can use shared folders of the operating system.
Keep in mind that some plugins will not work with older QGIS versions, so you might catch and install older plugin versions manually if you need them. And current QGIS project files might corrupt if opened with older versions.