You can get the stuff at any electronics type store but it won't be cheap if you get it at radio shack. I know it comes in 1/4, 3/8 and 1/2 inch diameters.
I have a Harbor Freight store near me and they sell in prepackaged bags. It like $1.99 per bag. You get 14 ft of 1/4, 10 ft of 3/8 and 7 ft of 1/2 inch in a bag. The stuff at Harbor freight is from PICO. The package says Pacific Industrial components, Inc, Eugene, OR 97402. I see you are in Oregon maybe you can call and ask who sells it near you.
You may find it at one of those electronics stores which sells lots of components. I do not recall seeing it at home depot or Lowe's but then it may be there and I overlooked it.