Filemaker 12 Trial Installer
For Filemaker Pro/Advanced 12 usually you'll have to contact customer service, provide proof of purchase and ask them to send you a new download link for the installer. (source) The good news is you get free support on installation as part of the license you pay for the software one time and you don't need to pay maintenance to get this help.

But here are links for
the Trial: Filemaker 12 Trial
Filemaker Pro 12.0.3.328 Filemaker Pro 12
Filemaker Pro Advanced 12.0.3.328 Filemaker Pro Advanced 12

For Filemaker Server 11/12 versions:  

Server Versions 11/12 open the blue triangle and click the "FileMaker [11/12]  Updater"-link. Scroll down the opening page and a full installer for "FileMaker [11/12]" will be available for download.