Yup, Paul is correct on the pounds. If you put in tank pressure without a regulator, you will be purchasing new shock parts.
I, too, use an old oxygen regulator from a welding setup. Works great, but you need to add an adapter to fit the bottle. My bottle is about 48 inches tall, and sits on its side facing the side of the van just in case the top gets broken off in an accident. In this event, the bottle will rocket sideways through the side of the van. Easy exit...
I usually refill my cylinder 3 times a year for about $20 each, but I also run my air tools off the nitrogen, as well as inflat tubes, so use it for more than just my shocks.
Good luck.
Rick