I have a Challenger panel board and one of my breakers is no good. When I went to buy a new one they don't carry them anymore. They said they only carry the after-market ones. So I got that one, but it looks different than the original one. Will it fit in even though it's a different looking one?
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.
You might want to think about replacing the entire panel sometime soon. Just Google Challenger, Stablok, Federal Pacific. Eaton makes replacement inserts to keep old box, but have new guts...main breakers, bus bars, circuit breakers etc. Old box, old wires new everything else.
It will likely fit & you probably won't have any problems. But, read the panel's door for specifics of a replacement. Some panels are very particular about what materials or brands of breakers go in.
If there's no mention of "only these brands", then whatever fits securely will be fine...they all have to meet the same standards, looks don't mean anything to electricity.