Answers:
USB or only external antenna?
For usb design try
http://www.nodomainname.co.uk/cantenna3/...
For can antennas try
http://www.turnpoint.net/wireless/canten...
Or if you freshly want to buy one
http://wirelessgardenstore.com/detail.as...
Hope that help.
Get a copy of the ARRL Antenna book. The information in it will assist you build what you necessitate