If you know the namespace involved, you can use it directly -- for example, if all classes are in module zap, the dictionary vars(zap) is that namespace; if they're all in the current module, globals() is probably the handiest way to get that dictionary.
If the classes are not all in the same namespace, then building an "artificial" namespace (a dedicated dict with class names as keys and class objects as values), as @Ignacio suggests, is probably the simplest approach.
