What are implementations of PRNGs based on ChaCha20? I thought I'd find an implementation in NaCl, but /dev/urandom is what I find.  Search engines samples revealed most implementations are broken.

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.

Bernstein's Fast-key-erasure random-number generators article is a good starting point, as well as how randomness generation is done in Amazon S2N.

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.