There are a few sites to find deals on Mac stuff...One being:
Apple also has their own "Hot Deals" website at:
Also, on The Apple Store's Website in the column on the right is a Button that says "Special Deals." Here Apple offers some goodies such as refurbished computers, iPods, etc, for discounted prices.
If you are in the education market, some special deals can be found here on Apple's website, also
This FAQ article may also help you is you are looking for systems:
Where can I find the best prices on New and Used Macintosh computers?