The first place to look is the Apple store, which is where you’ll be able to upgrade your Mac to have all the bells and whistles that you’d like it to have. Here, you’ll be able to choose to put more RAM, storage, or add in a more powerful processor. You’ll pay more for the privilege however, and even if you go for the base models in the Apple store you’ll soon find that things rarely move from their full price. Occasionally, you might spot a good price on a refurbished model, but those tend to be snapped up fairly quickly.
If you only need a base model or only need to slightly upgrade the RAM or the storage, then Amazon is a good option. There’s Prime shipping for next day delivery, and sometimes even discounts, although you’ll not have as many options if you want to choose what’s in your Mac desktop.
Best Buy have more configuration options, and you’ll also find that there are sometimes discounts as well. B&H Photo is another good retailer for this, where you’ll also find fast delivery options.
If you’re looking for a deal, then keep an eye for Black Friday. Over the weekend of Black Friday, some retailers put the Mac line at lower prices, and give out some excellent discounts.