They have source code files for their 'Freedom' core designs up on GitHub under an Apache 2.x license. These can be directly used to evaluate their designs as FPGA soft-cores.
SiFive has E-series (aka "Freedom Everywhere") and U-series (aka "Freedom Unleashed") cores, both of which seem to be based on Rocket. And they do provide high-level designs for both on their GitHub, under a free license.
I wrote SiFive CPUs, not RISC-V there. (because this BeagleV board uses SiFive)
There's a _huge_ gap between academic implementations and ones that are actually worth shipping. I can see RISC-V winning quite handily for microcontrollers. But on anything bigger than that...
Western Digital's open sourced SweRV cores are approximately the same performance as the closed-source SiFive 7-series ones.
The main difference is the WD cores are 32 bit, no MMU, probably DTIM rather than cache (not sure).
The guts of the dual issue decode, register file, pipeline is there, which is the hard part. The other bits could be relatively easily added by the open source community, probably cribbing components from RocketChip.
They aren't for the customers of SiFive. What is your greater point with this comment? That SiFive isn't Open Source or that they are equivalent to Arm?
This video from SiFive clearly explains the RISC-V ecosystem and how SiFive is involved.
As I said elsewhere, SiFive CPUs are just as closed as Arm ones, you just pay a royalty to SiFive instead of Arm.