There are many good places to buy computer parts. You can go to a computer retail store in your area. The support is usually good, and they provide decent warranties. The trade-off will be that you will pay a little more than you would in other places. Sometimes, a lot more.
Most towns have smaller stores that sell and repair computer equipment.
These are not large retail stores, but instead usually in an office suite in a strip mall. Regardless of location, though, such stores are often cheaper and can provide individual attention. Their hardware is often retail packaged from the manufacturer, in a box and all. But, they also sell OEM hardware, usually wrapped in nothing but plastic and have little documentation. You will need to be the judge on this type of hardware.