SiFive seems to be selling the only publicly available chips as of yet. The FE310 (~300MHz 32-bit microcontroller, FreeRTOS capable) can be bought on the HiFive1 and one other dev board, or as bare chips. They say they'll ship a FU500 (~1.5GHz quad-core application processor with supervisor mode and memory protection, capable of booting Linux and FreeBSD) dev board (board called HiFive Unleashed) this quarter.