Replacing a Challenger breaker with an after-market breaker
You can use BRs instead of Cs
If your panel labeling says it takes type C breakers, then there's an easy way out of it: type BR breakers (as depicted below, with the type C labeling highlighted) are legal, safe, and widely-available (at any big-box or hardware store) replacements for type C breakers, as they are cross-listed (as per this letter from UL to Cutler-Hammer, as reproduced at the bottom of this post) and cross-labeled to replace type C breakers.  (A more modern version of this letter is available, linked from an Eaton FAQ.)

For more exotic Challenger breaker types, Eaton cross-reference document TD01201041E provides the gory details.

(PDF version)