As far as I know, only the Freedom E310[1] chip is available for purchase by normal consumers. It's a microcontroller and only has 16kB of RAM so it's probably not really what you are looking for. But if you are still interested in it, you can pick up a HiFive1 devkit at crowdsupply[2] and possibly elsewhere. There's also the cheaper LoFive[3] which is a very bare breakout board for this chip, but I'm not sure if it's possible to buy it anymore.