Where to get Mavericks (or older) OS X installer apps to set up Virtual Machines?
There are many ways to skin this cat.

First, check to see what minimum version of the OS ships with your Mac:


https://support.apple.com/en-us/HT204319


That establishes the version you can get easily from Apple either through Internet Recovery or contacting AppleCare to order replacement media or a download code for the Mac App Store. At that point, the AppleID you are using should be able to upgrade to each of the versions of the OS newer than the build that shipped with your hardware.

To go earlier, you can join Apple as a developer and request download codes and direct download of all manner of older OS for testing purposes. If this isn't worth the time/budget to spend one time $99 you could also look for hardware that shipped with the version you require and buy it at a steep discount. For example, laptops with broken screens and failing batteries will work just fine with external keyboards and power cables in most cases long enough to get to the Mac App Store and "purchase" the version of the core OS for your AppleID. At that point, you can start building your archive of installers.

I would caution on torrenting OS unless you check the file sizes and hashes. It's probably better to network with other developers to exchange a USB drive with the media in exchange for brownies or such things like fermented grain based liquids.

Now that you have the ingredients, it's time to cook. Many open source tools exist to help you make bootable images:


http://www.theinstructional.com/guides/build-deploy-os-x-images-with-autodmg
https://github.com/MagerValp/AutoDMG
http://www.deploystudio.com
https://derflounder.wordpress.com/2015/10/18/create_vmware_osx_install_dmg-script-updated-with-el-capitan-support/
https://github.com/rtrouton/create_os_x_vm_install_dmg


My advice would be to pick one tool like Rich's create_os_xvm_install_dmg script or decide to set up deploy studio and/or a netboot server and then automate one of the newer OS. You'll then be able to not get hung up on gathering all the images and instead get going with the tools. You can ask specific follow on questions here or on the Mac Admin slack channel once you have your feet wet and are in the weeds of a specific install.