The genuine IBM-PC Speaker (Part#8529143) is rated as 8 Ohm 0.5 Watt. Back then a very common small speaker for cheap radios, cassette recorders and alike. I guess still easy to come by today.
It was driven by a SN75475 (or MC1472 see here at p.5-42) line driver (*1) capable of providing up to 300 mA which equals to a static load of up to 1.5W. The 33 Ohm series resistor limits current to 150 mA in case of shortcut or ~120 mA with speaker, allowing an effective output of 0.55 Watt, so still (basically) within spec.
*1 - While the basic PC design is quite crappy, individual parts are well made, not at all cutting cost. The IBM solution is quite reliable and safe. The Apple II speaker in contrast is driven by a Darlington transistor prone to burn out or unsolder itself - heck, I even managed once to kill its equivalent within the IOU of an Apple IIe.