Not possible if you pass in just the regular function since that will be a Function instance. You can get the JS source but I presume you want the CLJS source?
You could modify the place where you call it to pass addtional information so you can eventually get the source info but a regular passed function does not have the required info to get the source.
In this specific case I need to know the source of Clojure function, I suppose whatever solution is out there, it would be very different for Clojurescript.
For those wondering, as I did, what the difference between clojure.repl/source and clojure.repl/source-fn is, source is a macro that prints the source code of a given function directly to standard out, and actually uses source-fn, whereas source-fn is a function that returns a string representation of the given function.
Both require that the function be given as a symbol that resolves to a Var defined in a namespace for which the .clj is in the classpath. This is why it is necessary to first get the name of the function, demunge it (e.g. (clojure.repl/demunge "clojure.core$_PLUS_") => "clojure.core/+"), and then convert it to a symbol before passing it as an argument as @borkdude showed in his answer.