Common Words:
* Accuracy
* Adjacency
* Agency
* Concurrence
* Constancy
* Defiance
* Efficiency
* Elegance
* Frequency
* Infancy
* Insufficiency
* Latency
* Luminance
* Mercancy (archaic)
* Obscenity
* Pregnancy
* Redundancy
* Relevancy
* Transparency
* Vacancy
Less Common Words:
* Abeyance
* Aviancy
* Capriciousness
* Currency (also has other meanings)
* Decadence
* Deviancy
* Emancipation
* Exuberance
* Fluency
* Gaudiness
* Heresy
* Immanence
* Incompetency
* Insignificance
* Invariance
* Opacity
* Pristineness
* Regency
* Reluctance
* Resilience
* Succinctness
* Transparency
* Ubiquity
* Vengeance
* Vulnerability
This is not an exhaustive list, but it provides a good starting point. You can find more words ending in "-ancy" by consulting a dictionary or online resources.