The Regulator:
A regulator screws on to the cylinder and does two things: 1) it reduces the pressure in the cylinder to a useable level, otherwise the 2000psi tank pressure would just blow the exit bag off your head as soon as you opened the valve, and 2) it controls the rate of gas flow into the exit bag —specifically, it should release it at 15Lpm, which is the minimum adequate to carry away exhaled CO2 and fool your lungs into believing there's enough air.
The good news is that while gas cylinder sizes are not globally standardized, the cylinder valve threads are —and both N2 and Ar cylinders have the same threads (other gasses have different threads), so a regulator that fits on one cylinder will fit equally well on another.
You don't need to worry about being sure to choose a regulator that will reduce the pressure: they all do that. The flowmeter is the critical element of the regulator for our purposes.
Some regulators come with a "click adjust" flowmeter that is very simple, allowing you to dial in 15Lpm without needing to think about it. To the best of my knowledge, these are medical grade regulators; generally very good quality, but often a bit more expensive. Others here may know more about them, and where to acquire them, than I do.
A typical welding regulator will have two gauges: the tank pressure gauge (unimportant to us) and the flowmeter gauge.
A welding regulator's flowmeter gauge will be marked in either cubic feet per hour (Cfh), Lpm, or both. Some welding systems require a fairly low flow of inert gas, much lower than our necessary 15Lpm, and this low flow is most easily measured in Cfh. If you see a gauge marked only in Cfh, it probably will not work for our purpose. Look at the highest Cfh setting on the flowmeter and do the math, to be certain. If the gauge reads in Lpm, a glance should tell you whether the regulator provides the necessary 15Lpm flow.
Harbor Freight Tools offers a cheap CO2/Ar regulator (it also works for N2) with a flowmeter that shows flow in both Cfh and Lpm and goes high enough for our purposes. This is what I have.
The Hose:
The hose needs to be long enough to reach from the gas cylinder beside you, up into the exit bag on your head. I have found it easiest to lead the hose up my back and into the bag at the very back of my neck. It seems to disrupt the fit of the bag less right there.
Some regulators come with a hose that threads directly into the regulator body. Some regulators come with a hose-barb, onto which a length of soft tubing can be pressed. Some regulators come with neither.
For the first situation, just thread the hose into the regulator, tighten it with a wrench, and it's ready.
(Regardless of what type of threaded fitting is used, you do need to wrench-tighten this fitting: a lot of gas can leak out at this connection. If it is a brass fitting, you should not need teflon tape on the threads; the soft metal deforms enough when tightened to provide an adequately gas-tight connection.)
For the second situation, take the hose-barb to a hardware / home improvement store and purchase tubing that fits onto the barb. Once home from the hardware store, thread the barb into the regulator body, tighten with a wrench, and press the tube onto the barb. It should be a snug fit; if you're worried it's too loose, use a small hose clamp / jubilee clip to secure it in place. If it's a little too tight to get the hose into place, soak the end of the tube in very hot water to soften it and press it onto the barb. I used clear vinyl tubing. Aquarium tubing should work. Surgical tubing may not; I don't know for sure.
For the third situation, you'll need a hose-barb fitting (typically brass) that threads into the regulator body. Take the entire regulator to the hardware store and ask someone to help you fit it with a hose barb. Then proceed as for the second situation.
Cover story: as with purchasing the gas cylinder, you're using N2 for brewing, or Ar for filling partial paint cans. Why Ar? because it's heavier than air and settles down onto the paint surface inside the can. For extra confidence that they won't ask inconvenient questions, you're doing it for your father, or your brother, and you don't know why they want it for their paint locker, they just said to get a hose barb and six feet of tubing…
The Exit Bag:
There are several videos on YouTube detailing construction of exit bags. I strongly recommend watching them. That's how I learned.
The short version of what you want is a turkey roasting bag —available in the supermarket, in with the zip-locks and cling wrap— with an elastic drawcord worked into a turned hem, and a cord-lock to adjust the drawcord's tension.
I used 3/8" elastic cord. I purchased both it and the cord lock at a sewing and crafts store (JoAnn's).
To construct the turned hem you'll need micropore (surgical) tape, available at a pharmacy in the first aid section. It's the right tape to use because it will reliably stick to the material of the bag itself. Sometimes it isn't labeled as "micropore." Ask a clerk if you can't find it.
Cover story: you were asked to re-stock the family first aid kit with micropore tape and you don't recognize any of the brands available.
Using an elastic drawcord is important: it needs to fit snugly around your neck, but not seal tightly. It must still be flexible enough for the flow of inert gas, lightly pressurizing the exit bag, to push past the elastic and flush away the CO2 you're exhaling. Don't use a non-elastic drawcord or do something like duct tape the bag to your neck; you don't want to create a seal, just a restriction.
