The first port of call is going to be the Apple website, where you’ll find all the configuration options. This is the best place to go if you want to spec up a Mac and put more RAM, storage, or a more powerful processor inside. One thing worth noting about the Apple website, however, is that there are very rarely any kind of discounts, if ever. Students get slightly lower prices, as do teachers, but the Apple store is still going to be full price for most of us. From time to time refurbished models appear on the Apple store, but you won’t have any say over what configuration these come in, and they tend to be snapped up quickly.
If you don’t want to make too many changes to the base spec of a  Mac, then you can get one from Amazon. There, you’ll see discounts and fast shipping, albeit with fewer options to dial in exactly what you want from your Mac. There are other shops where you might find a Mac with more options, however.
BestBuy has configuration options, letting you choose different components that you may want to put in your Mac. They also have discounts as well, so you might even save a buck if you shop there. The same can be said for the retailer B&H Photo, where you can also choose what you want from your Mac. Fast delivery is also available here, and there are also discounts on occasion.
If you’re looking to save money on a Mac, you should watch out for sales like Black Friday. At some of the retailers above, you may find some brilliant prices on Macs over the biggest sale of the year, meaning you can get more Mac for your money.