http://www.omega.com/pdf/tubing/coup...fc12series.asp
I was trolling and knew the source.
The spec pictured is for the 3/8" ones. I use the 3/4" W 1/2" ID Tygon R3603.
I installed them as TBirdX did. I don't have to drain the system or have two people to move it. They are fairly restrictive, but I think they can be modded to be better although I havn't had time to try. In a system with an Eheim 1048 they reduced flows from 0.91GPM to 0.78GPM.