We go through a lot of cables: Ethernet, USB, HDMI, Lightning, coax; you name it. People often ask us where we get them, especially when we give them away (which happens a lot—we don't want to charge extra time for a job simply because a necessary cable wasn't available).
We purchase our cables from Monoprice.com. They offer a lifetime guarantee on most cables, and we've rarely had an issue (when we did they sent us a new cable immediately for no charge). They're also crazy cheap. An Ethernet cable that costs $20 at Best Buy can be purchased from Monoprice for around $2. Plus, they are available in a plethora of colors, so you can match pretty much any decor.
Two things to keep in mind when buying cables:
The more expensive Ethernet cables are shielded, which is important if you're running them alongside other electrical equipment or coiling them up into loops.
Don't spend extra on HDMI cables. Any claims about how they improve picture quality are nonsense—it's all digital, ones and zeroes. You can't get part of a 1 or an O instead of a 0. The system uses error correction, so if the information is wrong it just gets it again. As long as it works at all, you're generally good to go.