Several users have reported their favorite places to acquire Mac gear. Apple sets a minimum retail price for all authorized resellers, so the prices should be about the same everywhere. Best advice if you are looking online is to find a place that doesn't charge sales tax in your state, and look for the store with the best deal on shipping. Many places also give you a discount when you bundle items together (Displays, iPods, etc). College students may qualify for educational discounts as well.
If you are looking to buy a New or Used Macintosh computer, keep an eye on LowEndMac's "Week's Best Deals". They track the lowest prices on used and new Macs from Mac dealers. They highlight the lowest price for each model of machine, and they categorize them by machine type. You can find an index of all of the current week's Best Deal pages here.
Each week, each page is updated and the sites are scoured for the latest low prices. This is a great resource if you are looking to buy a new or used Mac, and don't want to do all of the price comparisons yourself.
There are a few sites to find deals on Mac stuff...One being: DealMac Apple also has their own "Hot Deals" website at: hotdeals.apple.com 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.
Check out the Mac Buyer's Guide from MacRumors. It offers a synopsis of each model Mac and offers advice on when to buy based on average product life cycle, pending updates, and possible related rumors. Most of the recommendations are based on the average number of days since the product was last updated and how many days have passed since the last update.