it depends. What do you want to do with it? Are you searching for an already existing instance of the Class or do you want to create a new one?
If you are searching for an existing instance, you should already have it referenced somewhere, or in worst case can iterate through all the Actors in the Scene implementing your Baseclass.
If you want to create a new instance, i.e. for spawning a Mesh or Particle-System or stuff like that, you could use StaticLoadObject:
Cast<MyBlueprintClass>(StaticLoadObject(MyBlueprintClass::StaticClass(), NULL, *mypath));
see also the page in the wiki.