I've made alot of soda over the years, and I found that I needed 30' lines to make the soda retain carbonation. 30-40 psi is the right pressure at fridge temps- I like mine at 30 psi, but I know David_42 said that his is more like 40 psi.
I bought some cheaper tubing, just for soda, from McMaster-Carr. I don't remember the model number, but I found it searching this forum. It was like 15 cents a foot- so much cheaper than beer line, but still food grade.
It was semi-rigid, so it was hard to work with, but it coiled up beautifully on the keg and that was a big plus.
Broken Leg Brewery
Giving beer a leg to stand on since 2006