Last night I spent quite a lot of time reading about this and I think, if what I read is not wrong, that a good co2 tubing has to maintain its shape, has to be inert (no reaction to co2) and should be flexible.
It seems that silicone tubing meets all this, but you can find people that have used this material without reporting any problem and other people say that co2 leaks in this tubing. Who knows... No one measures the co2 diffused. Air tubing is said not the right one, don't know the material but probably silicone also or other plastics (some of them cheap and smelly).
The only material with which everyone agree to be perfect is polyethylene, sold as co2 tubing in local pet shops.
In my town both tubing can be found very cheap in specialized plastic shops, in the length you want and in a wide range of diameters.. At least 3 times cheaper than in local pet shops.