The dark (or hidden) photon is a massive U(1) gauge boson theorized as both a dark force mediator and a dark matter candidate. Dark photons can be detected with axion cavity haloscopes by probing for a power excess caused by the dark photon’s kinetic mixing with Standard Model photons. Haloscope axion exclusion limits may therefore be converted into competitive dark photon parameter limits via the calculation of a corresponding dark photon to photon coupling factor. This calculation allows for an improvement in sensitivity of around four orders of magnitude relative to other dark photon exclusions and may be attained using existing data. I will present this conversion and limits on the kinetic mixing coefficient between dark photons and the Standard Model photons based on existing haloscope axion searches. These limits can be further improved upon without lowering the sensitivity of the haloscopes to axions by modifying the angle of the cavity and the measurement time. I will also discuss how new haloscopes being developed for more sensitive axion measurements will improve dark photon exclusions. (arXiv:2104.09334)