If you have installed the Find My iPad app from Apple (which is free), you can go to any computer with an internet connection and go to the Apple iCloud page to send an alert to the iPad. This will only work if the iPad is not completely powered off.
No, there is not at least one iPad in ever house in America. However, there is an estimated 55 million or more that does own an iPad.
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There will be a red dot on the top right part of your ipad
A website that can help you find an Apple iPad would be the actual site where Apple iPad's are manufactured. Also, many electronic stores sell Appple iPad's, like NewEgg.
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You can't find it using the serial number, unless you use it to identify it in found items. Find My iPad and iPhone apps will show you on a map where the iPad is if it turned on and has not yet been wiped.
You can find many of these iPad keyboards at very good prices. There is one from Kensington that is included in the iPad case. It retails for about $100 - $120 but you can find it cheaper on amazon or eBay.
There are about 3 or 4 places that offer IPad data plans. You could check your local AT&T or Verizon's service and request the iPad data plan for your iPad.
Ebay.com
In iBooks if you have an IPad