I have used both the standard medical supply house $2 cannulas and an aviation conserving $40 cannula. They both seem to work fine. I haven't noticed a significant difference in oxygen consumption between the two. A google search should show one close to your location. With a finger oxygen meter i keep track and adjust flow rates accordingly
I just get them from Amazon. I also bought the tank and regulator there. I have a couple of cheap Amazon/China pulse oximeters...they work fine, are reliable and accurate but they're inconvenient for continuous use so I bought a Wellue pulse oximeter, which fits on a finger like a ring and can be comfortably worn continuously
With EDS, they tell you to use specific a specific cannula type: "Do not use standard medical cannulas as you may not receive adequate oxygen flow or oxygen conserving optimization." I thought it messed with the triggering but haven't tested it for myself.
If that's not true I'll look into getting some medical ones at least for one-off passengers - the EDS ones are $11 so not terribly expensive.
