I would like to get ASP.NET Core/.NET Core Preview 5 source code, but in the release page there are no reference to the sources, and in the github repo I can not find any branches which name hints that it would be.
ASP Core Preview 5 release is available here. In general, you can find the branch/commits etc about the releases from here.
Similarly, for .NET Core use this link for Preview 5 and this link for all the releases