You can buy an iPad 4 through the Apple store. Otherwise you can shop around at the following authorized dealers:
For Consumer Sales
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Abt Electronics
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CDW Computer Centers
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MacConnection
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MacMall
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MacZone
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PCConnection
For Corporate or Government Sales
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CDW
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CDW-G
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ePlus
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GovConnection
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Insight
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MacConnection
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MacMall
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MILESTONE Technologies, Inc.
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PC Connection
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PC Mall
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PC Mall Gov
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More Direct
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Sarcom
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SHI International, Inc.
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ZONES
You can also find them at Best Buy, Wal-Mart, some Target stores and Radio Shack.
Resource: Apple