Depending on the set up of your system you may already have one. Any laptop in the last few years will have a WiFi dongle inbuilt as will many desktops. You have already got a wireless router so all you need is a wireless card for either your desktop or laptop (as you never specified).
For laptops click above. However you will need to consider what type of card your laptop supports (see the manuals) as there are 2 formats of cards.
Click this link if you are using a desktop. However be sure to find out if you have a PCI slot free to use. If not consider removing the modem if one is fitted in a PCI slot.
Click this link as these are universal to both laptops and desktops. However require a free USB slot and often cover USB ports beside the one it occupies. However most will come with a USB A-B cable to allow you to connect it without it sticking out the side like a sore thumb.
Finally ebuyer have some clearance items that may be cheaper so have a look there too depending on your final choice.
The last thing I will say is make sure to get a minium of a card rated to utilise G (often referred to as 802.11g). Ideally getting one that offers the N (referred to as 802.11n or 802.11 draft N). This will future proof you and also potentially give you better results.