In a word . . . mortars. I've tried with no luck to find a source here in the States for the rocket tubes that Duncan Price uses for his D5 rebreathers. However, through the course of my surfing sessions, I've come across a nice source for 5" diameter HDPE tubing in reasonable lengths at a reasonable price. 005-5009 - 5" HDPE Mortar Tube DR21 Mortar, 28" length (4.95" ID)

Of course you still need to make up some end caps, etc. That shouldn't be too difficult, as HDPE plate is cheap and easy to come by in good thicknesses for making end caps. (Modern Plastics, Interstate Plastics, K-Mac Plastics). This is a good start and a single purchase will make at least two scrubber tubes.

Hope this helps.