Typical users, including trial users do not need the source code. No source code is available for bug-fixes and development snapshots (developers have the necessary SVN access).
We understand that HPC administrators sometimes want to recompile and -link from sources. We can sometimes allow temporary source code access for that purpose. You need to request a source code license to download the source code (2018.105) with a source code license. If you do compile from source, also check out the platform specific information.