I've been using the O2 kit purchased here along with a Harbor Freight welding reg for the past 8 years with good success.  The "bag" has always been an inconvenience, having to constantly tweak the flow based on breathing pattern (i.e. periodic holding) and more than once falling back to sleep and emptying the entire cylinder.  I've always wanted a demand reg, but couldn't stomach the cost! 

This time around I decided to do some deeper shopping research, and was able to piece together a package including the demand valve 2nd stage with 6' hose and a 25lpm CGA-540 1st stage for $286 shipped!  Received the parts in about a week and have been using the system for a few days now.  The mouthpiece or mask from the O2 kit fits the reg. 
It's the "cats meow" so far!

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I don't have enough posts to add links yet, so add the  w's in front of the following-

The first stage came from:
...healthproductsforyou.com/p-responsive-respiratory-ems-oxygen-regulator--25-lpm-cga-540.html

The demand reg and hose came from:
...diversalertnetwork.org/dive-store/?id=39

Post if you can find either cheaper!

Great find on the demand valve! I'll be bookmarking that for future use.

Great find and thank you for posting it here so that others may benefit too.