I'm going to assume you have a USB port on your computer. (if not normally their is a special adapter in the box that you can convert a ps/2 to usb)
Just head to your favorite electronic store that sell wireless mice.

and just plug it in.
before you can use it though
there is normally a button on the bottom of the mouse (you might have to use a paper clip to press it) and there should also be a button on the receiver (the part you plugged into the computer). Press the button on the receiver and then press the button on the mouse. This enables the two to get in sync.
You should be good to go now

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Oh btw while you can have two mice connected at the same time. try not to