Where should you buy car parts? It depends on what you need them for, generally speaking, and how much time you have before you need them.
Regardless, it’s good to find the part number — preferably from the old part. Most of them have a number stamped or inked on. The full part number can change and be superseded by a new, improved piece; some companies append the number with letters to show revisions. This can be very handy for saving time, but can also help where an automaker had numerous different versions of the same car, each with its own special pieces.
For suspension or brake parts, I prefer to return to the manufacturer, even though few of them actually make their own. To a large degree this is a safety issue, particularly with regard to brakes, but for suspensions, there’s a second reason. An automaker may have twelve different part numbers available for different versions of a car, while the aftermarket has a single number. That’s because each model or trim of a car may special tuning, for different powertrains and such; the aftermarket may focus solely on whether the part will fit, which can result in some odd ride-and-handling changes. That’s especially true if only one part of a system, such as a shock absorber, is replaced. Even the most reputable companies do this — even the ones who supply the automaker in the first place — to lower their inventory levels.
Finding the part number on the old part helps to locate the right new part
If I have time, I prefer to buy from an Internet dealer, at a discount of anywhere from 20% to 40%. It usually takes them a good long time to ship, and they don’t really take returns (or, rather, they take returns, but under absurdly poor conditions). There are other issues, including clunky back-end systems that might not always work. The same dealerships often sell on eBay, WalMart.com, Amazon, and Sears.com, sometimes with better conditions, or at least easier comparisons to other shops. Just make sure you are getting true manufacturer (OEM) parts, because there are a lot of fakes out there; that generally means buying from a dealer. (Unpaid endorsement: I’ve done a lot of business with Koller in Virginia Beach, which does Chrysler, Ford, Kia, and Nissan. Like most of these dealers, they only give the discount for mail order. If you show up in person, you won’t get the discount.)
Spark plugs and many other maintenance parts are invariably made by a few companies you can buy from yourself, anywhere, and save a good deal of cash. I’ve switched from getting Trico blades to “store brand” from AutoZone and such, because they seem to last as long and work as well. They do seem very prone to dry rot, so age is a factor. I used to rely on Sears for batteries, but relying on Sears for anything now seems foolish; that said, when buying a battery, it’s worth spending more. The higher quality batteries can literally last years longer than the cheaper ones… speaking from experience, again. I’ve yet to have a good battery die before six years were up, while AAA apparently thinks three years is the norm.