Aquacave has blue silicone airline tubing for cheap and it is great quality. I use it for everything but the tubing in the doser head. I use lab grade dosing pumps so I cannot say if it'll fit in your doser head. http://www.aquacave.com/Air-Line-per-foot-P151.aspx $0.19 a foot
If you measure the exact size, there is a large selection of lab L/S dosing tube. You can reference my info for flow rates, sizing and part numbers. http://www.basementreeftank.com/9.html