Welding stores mark up everything really high.

I would suggest you look for a place that sells and services fire equipment. They will sell used fire extinguishers at a reasonable price. I picked up a 5 pounder for $50 and a 10 pounder for $65. You can get a regulator with gauges from Amazon for $20-40 that is used with keg fridges. It helps limit the amount of CO2 that comes out and makes it safer to use.