If the reader is near a 110/220 volt power source then an AC-DC power supply is a good solution.

There are two fundamental types of power supplies, linear and switching.  Linear power supplies apply a variable resistance to regulate the voltage.  Switching power supplies use oscillators to do the conversion and this can cause interference with the low frequency RFID signals.

Linear power supplies should be used and switching power supplies should be avoided. 

Our 4 amp linear supply can drive one reader but if you want to connect multiple readers to one power supply, one with a higher amperage rating is needed.  An 8 amp supply is sufficient for two readers, 12 amps for three.

Tripp Lite makes a wide range of linear power supplies that can be ordered from Allied Electronics.