There are many types of antennae for wifi.
Here is a link for a commercial manufacturer but they do have a large assortment of antenna's and methods of attachment from RF coupling to direct connect. The most common type of connector these days is the MMCX or MCX style but you have to know which connector you have to mate with the antenna.
http://www.radialllarsen.com/mobile.shtml
This Link below is a 3.2MB PDF document
http://www.radialllarsen.com/docfile...WLANMobile.pdf
The biggest problem is with the new laptops and wifi devices are the antenna is built into the plastic housing. Unlike the old cell phones you could remove the antenna and connect your own external (or they provided a jack to do so). Now a days, its getting harder and harder to find devices that have connectors on the antenna. So about the only thing you can do is search for a device that has a replaceable antenna.
Years ago you could just buy a Linksys or Netgear wireless router and connect your RJ45 cable and use an external antenna on the router. But, in their cost saving manufacturing process they have eliminated the antenna connectors.
Unless you modify your device and install your own connectors.(my philosophy, if it was put together, it can be taken apart)
The easiest home brew antenna is the pringles can wifi antenna.
Here's a link for your benefit with pictures.
http://www.oreillynet.com/cs/weblog/view/wlg/448
This is the commercial version of this antenna.
http://www.cantenna.com/
Here is another commercial site
http://www.antennasystems.com/broadb..._antennas.html
The antenna with the highest gain # will capture more signal but your aiming of the antenna is more critical due to smaller beam width.