Where's the best place to download Ubuntu virtual machines?
The image file downloads for the latest version(s) of Ubuntu (including 14.04) are listed on Ubuntu's Release/Downloads page (searchable in Google).

Here's a guide on how to install the system from the ISO format, using VirtualBox (your tool of choice):

http://www.engadget.com/2009/09/07/how-to-set-up-ubuntu-linux-on-a-mac-its-easy-and-free/

For the VirtualBox image (instead of ISO), go here (lists versions from 14-15):

http://www.osboxes.org/ubuntu/

As such, it goes without saying that you should use the original source whenever possible.

Good day to you!