Is Mojave upgrade no longer available ?

I run High Sierra and would like to upgrade to Mojave. All the links which have been quoted in various Support postings lead to Mojave 10.14.6 and this cannot be installed without the basic 10.14. All previous updates 10.14.2 to 10.14.5 are still available but none of those will install on their own either - for the same reason. Surely this is a mistake ? Or have Apple made a deliberate decision not to allow it.

Is it safe to use other unauthorised sites to install ?