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Try newegg.com or macsales.com (OWC). They both have great prices.

I got my 320g WD for my Macbook at newegg for $130 a few months ago. They also had fast shipping, too.
 
If you are planning on upgrading the HD in your MBP you may want to consider buying a Western Digital Passport HD because they come in an enclosure that you can use to put the old HD in.
 
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