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If you are brave, ebay has the best prices. I got the 2.6, 8, 512 for $2,450.
 
As far as a new unit, probably Amazon or Best Buy. Apple Store is a good deal if you are a student as well.
 
Best Buy stores collect sales tax. The best price I found for my rMBP was MacMall. No shipping, no sales tax, and 3% off. The only thing better would have been to qualify for ed pricing on a tax free weekend at the local Apple store. The MacMall price was cheaper than ed pricing with local sales tax.

http://appleinsider.com/mac_price_guide/
 
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