You can get the game port cable connector for your motherboard from several online vendors, do a google search for "game port connector" or something like that. here is one vendor that has a game cable (this one also has good diagrams of the pin ID's incase you motherboard has a nonstandard game/MIDI plug (or header), which will allow you to connect the correct pins to your motherboard pins as long as your motherboard manual tells you the Pin ID's on the motherboard are):
http://www.frontx.com/cpx091_2.html
with this particular model, you can mount your joystick port on the front of your computer in an empty 5.25 inch bay (see
http://www.frontx.com/ for the 5.25 inch bay module that allows custom port configurations on the front of your computer case)
usually the game plugs (headers) on most motherboards and soundcards are a standard 2x8 header connector, meaning their are 2 rows of 8 female pin holes. If your motherboard has this, just make sure you match the pins correctly.
This next vender has a less costly option ($4.50) if you dont care about mounting the game/MIDI port in the front of the computer. It mounts in the back of the computer on a standard PCI slot cover.
http://www.mycableshop.com/sku/PLATE15F.htm
cheers....