I've read a great deal about the development of Germanium field effect transistors and HJTs but are they actually been manufactured commercially? If so where can I buy them? I already know where to get Germanium diodes but I want MOSFETs if they're available.
Short answer NO
These are still in the research/development phase and will not be available for purchase any time in the near future. If you see them available for purchase, they are quite likely a scam or unreliable. The only people I forsee selling these in the next 2 years also sell fake Persian rugs.