How can I find a wifi solution for older Mac Mini?

First of all, my apologies if this is an off-topic post, but I assume there is some great wifi knowledge here, so perhaps I'm not too far off the mark? Here's my dilemma. I am moving my Mom's old Mini into an office that isn't wired for cable or Ethernet. Her Mini did not come equipped with an internal Airport card, nor any of the antennas and assorted internal goodies that came with the machines equipped to be wireless from the factory. My idea was to use a USB dongle rather than look for all of the parts I'd need to do something internally. The trouble is that when I go into the local Best Buy, Staples, Office Depot, etc., there appears to be nothing I can purchase off the shelf that works with a Mac out of the box. Can anyone tell me what I need to look for as far as a make and model that will have drivers which will work for a G4 Mini running OSX 10.5.7? Will I see a drastic reduction in speed as compared to the current Ethernet-tethered configuration? How about those Ethernet network extenders that plug into an outlet and utilize the electrical wires in the home as their medium of data transport? I don't really know what direction to go in to solve my problem. What products/methods would you choose, if this was your dilemma? Thanks in advance for any wisdome you care to impart.