Noibat can be found in Terminus Cave or Victory Road in both X and Y.
When you enter these caves there is a chance that a Pokemon might swoop down from above. This is like the dust clouds phenomena in Gen V. You don't know when it can do that... but, yeah. Just run around hopelessly in either one of them caves and hope you get swooped by a Noibat! xD It's quite rare, by the way, so be patient.
Hope I helped. :)