https://dev.sifive.com/freedom-soc/evaluate/fpga/
The FPGA board can be used for other things too. If you are really adventurous, I'd suggest buying a FPGA board with better chip so you can fit in larger IP blocks in the future. It will work perfectly fine as a replacement for the above FPGA kit I've linked to.
My suggestion would be this[1]. It has a pretty large LUT count so you can go nuts. The RAM and Ethernet will be pretty useful if you want to run linux[2] and test out stuff. It'll be a bit hard to run linux on it right now.
On the other hand, you can choose a Parallela board[3] which comes with a FPGA chip along with a new (soon to be retired) arch called epiphany. Here[4] is a GSoC project which runs linux on that board using the FPGA.
[1]: http://store.digilentinc.com/nexys-4-ddr-artix-7-fpga-traine... [2]: https://github.com/riscv/riscv-linux [3]: https://www.parallella.org/ [4]: https://github.com/eliaskousk/parallella-riscv