What libraries are there currently? (I'm only interested in the PRNG based on ChaCha20.)
Beyond what specific primitive your system will be based on, there are important things to consider while building a CSPRNG.
Buffer output, but erase data as it's being read, frequently rotate the key, mix multiple entropy sources if possible.
Libsodium provides a Salsa20-based CSPRNG implementation. It is not used by default, as for most applications, reading
/dev/urandom is perfectly fine.
ChaCha20 can be a drop-in replacement for Salsa20 here.