If you are supporting a desktop you'll be looking at wireless bridges
(DWL-800AP or similar that can be placed into bridge mode) and if you
are supporting a laptop look at USB adapters. Once you've made that
choice... toss a good "home brew" reflector into the mix and you should
be able to accomplish what you want.
Look here:
http://www.freeantennas.com
and here:
http://www.usbwifi.orcon.net.nz
When building homebrew antennas stick with the easy to build, cheap,
designs first because you can throw them out if you don't like them.
Stay away from the recommendations that the vendors who frequent this
forum give you about buying a commercial antenna. That is almost
always completely unnecessary.
-m-