Why buy a cable modem? You can rent the modem for $3 a month from Comcast. That will take three years to get to $100. By then who knows where you will be living (if cable is available, etc.) However, if your really have the urge to buy a modem, go to any online store (Amazon, Best Buy, etc.) and get one. Motorola and Toshiba make some of the best ones. Really, they all work well so pick one of those two brands.
Also, I saw a Best Buy ad that gives you the modem for 100 bucks then a $100 mail in rebate when you order comcast internet. You'll have to check Best Buy (There site is down as I'm checking right now).
Like someone said above, you will need the modem -> wireless router -> wireless card tomake this all work.
UPDATE: I just got in to
bestbuy.com . Scroll to the bottom of the page. Under store services you'll see a ad for "weekly ad". Click on it, ebter your zip code. When you get to the next page, search for cable modem. You'll see the Lynx cable modem and a link on the side that says "view page 19" (your might be different depending on your flyer for your area). If youlook at the ad, if you sign up for comcast, you get the cable modem for free, plus a $25 gift card from best buy. And there you have it!