The chips used in the hifive board are not available for sale anywhere. They're apparently a one-off run for the hifive board specifically.

Therefore, they're not a good choice to base a project on.

I do look forward at low power microcontrollers based on risc-v with some actual commitment to continued availability.

The hifive is marketed as an arduino-like, but at 320mHz it seems like it should be able to run a stripped-down Linux distro. Not fast, but if Linux can run on puny MIPS boards...