You have a couple of options available to you. You can try and track down a CH Products Gamecard GC300 (
http://www3.shopping.com/xPF-Ch_Products_CH_Gamecard_GC3_300_054~r-1~CLT-INTR) and install it in your system.
Alternatively, and depending on what type of sound card you already have, you can consider going with a Soundblaster Live value card very inexpensive), which has a gameport on it. This, however, is only a solution if:
a. You have onbaord audio and are willing to disable it (easy to do) on your motherboard; and/or
b. If you are willing to part with your existing PCI audio card and switch it for the Soundblaster Live value
Others may disagree, but I would probably go with the Soundblaster Live value, which sells for about $10.00 (USD).
Hope this helps.
John