The cheapest option on ThinPenguin currently is that USB adapter ($21.97 +
taxes + shipping):
https://www.thinkpenguin.com/gnu-linux/penguin-wireless-g-usb-adapter
You can as well search for used devices where the seller indicates the
chipset (typically with 'lspci' if it is internal, 'lsusb' if it is
external).
You can buy this one, that I use myself (plug it: it works!), for less than
10€:
http://www.ebay.it/itm/Wlan-USB2-0-TP-Link-TL-WN722N-Adapter-/360911237623?pt=Wireless_Networking_WiFi&hash=item5407fc71f7
