"Radio Shack" and "Good Coax" usually are mutually exclusive. Radio Shack doesn't sell good stuff, but "good" is relative.
If it's 75 ohm cable, it's likely either RG-59 or RG-6, depending on when you purchased it. It'll say on the cable jacket. Knowing what it is will help….
Beware of people trying to tell you that 75 ohm coaxial cable will not work with a scanner or scanner antenna due to the radios/antennas having inputs designed for 50 ohms. It really doesn't make a noticeable difference.
So, your cable/antenna:
1. How much cable do you --actually-- need? Using just the amount you need will help reduce cable induced signal losses.
2. Is your antenna mounted above the roofline, or is it located inside a building? If it's inside, what is the roof made out of?
3. How high can you -safely- get your antenna? Higher is better when trying to improve coverage. The higher it is, the farther away the horizon is.
Good coax:
Depends on the length you need. If it's a short run, less expensive cable will work just fine. If it's a longer run, you will need a higher grade coax.
Radio Shack RG-8 is about as good as you'll find in their stores. It's not great stuff, but if it's all you can get, it's all you can get.
A better choice would be to purchase Times Microwave LMR-400 cut to length and professionally terminated. There area number of sellers on the internet that will do this for you. Make sure you are getting -real- Times Microwave brand cable, or one of the bigger name brands. There are a lot of dealers using cheap Chinese knockoffs that do not perform as well.
Important:
LMR-400 and similar cables are stiff. Do not attempt to connect them directly to your scanner if at all possible. It will put a lot of stress on the connector on the radio, and will risk breaking it. Standard procedure is to use a short length of smaller, more flexible coaxial cable to make the transition to your radio.
So, recap:
Good Yagi antenna up as high as you safely can, out in the clear.
Good coax, as short as possible (no extras rolled up somewhere wasting signal).
Properly installed connectors to match the antenna.
Flexible jumper at the radio end.