I presume you are referring to the standard push button type of lock which has a plastic body and normally a brass push knob. If so these are readily available from most good chandlers as they are always jamming and need to be replaced. The UK chandlers "Force 4" carry a good stock with a variety of trim fittings.
For a stuck door a sharp blow on the door, in the vicinity of the lock, with the palm of the hand sometimes works.
Once freed they can be stripped and cleaned and then are best assembled with no lubricant - oil is what causes them to stick in the first place.