Where can I buy a Wifi SD card for my cell phone?

This handy little device allows you to connect your cell phone into the Internet anywhere you can get a WiFi connection. It will even allow your phone to connect to your home computer!
Since you didn't specify which phone you have, and these come in many different specifications, I've recommended a few of the top sellers for you.
The Palm 80211B Wi-Fi card allows you to connect anywhere you have Wi-Fi access - at home, the office or at hotels, cafes and airports using Wi-Fi (802.11b, channels 1 - 11) technology. Send and receive email, browse the web, even wirelessly synchronize with your computer. It adds network connectivity to the Tungsten T3 and Zire 72 handhelds, conserves battery life with low-power standby mode and includes access to corporate networks.
The SanDisc Connect Wi-Fi SD card is the smallest Wi-Fi card in the market. The card will allow SDIO enabled devices to connect to HotSpots (802.11b AccessPoints) worldwide for fast wireless Internet connectivity. The card supports Pocket PC 2002, Pocket PC 2003, Windows Mobile 2003 and PalmZire 71 devices that have an SDIO enabled slot.
For much less, http://www.buy.com has the Spetec SDW821 SD WiFi 802.11b/g card. Spectec SDW-821 is the only SD WIFI card that supports 802.11b and  802.11g on the market. This SDIO Wi-Fi card is compatible with today's most popular PDA models: iMate, iPAQ, Dell, etc. The card's sleek and compact design is less than half the size of today standard SD wirelesscards, while outperforming its competitors in stability, speed, range,reception strength and more. It gets great reviews and costs just $47 with shipping. It can be found here.