You have several options to power these radios, including repurposing an old canopy power supply for the injector end (just cut off the power brick). Packetflux's Gigabit injector can also power these radios with your choice of power source as long as you are within the 22-30VDC range. I would ensure you are using regulated power source to prevent over voltages from reaching the radio.
A word of caution though: some aftermarket injectors can not be made to work with the PTP450 series, please ensure the manufacture actually supports that particular radio with their injector.