Eight free online courses
The good news is that they are free; the bad news that are listed in a web page of 700 free courses. Still you might find something else that you are interested in!
If you search for C followed by a space (Ctrl+f in your browser on the web page) then there are only 35 hits, which means only 5% of the reading. There’s also a C++ for C programmers course in there as well.