Unless you're shooting one of the big bire (9mm / 45 cal) airgun... having a mold "just in case" is kind of a moot point.
- .177 and .22 are good for small games.
- A sleeve of 5,000 pellets will cost you less than a mold. And since you won't use that rifle/pistol to fend off zombies, 5,000 pellets should last you a long time.
If you're into bigger stuff, this guy's stuff is often talked about. It will set you back $200 + shipping + the regular casting stuff
http //nomad-pelletmaker.weebly com/