Hello,
The Scout camp that I worked at several summers gets their Oxygen from the local welding shop which also supplies medical grade Oxygen. The tanks, regulators, masks and other such they bought from Moore Medical.com Moor Medical. An Aluminum D tank is $71+s&h which I think is what our camp got. You may not need a tank that large in which case you could look at the Portable Life-O-Gen Life-O-Gen. I'm not sure what there official shelf life is but I have heard people complaining that it is only a few years.
I hope this is of help to you.

Zechariah