Look in the Yellow Pages under "Welding Supplies."
Also, eBay carries tanks, regulators, masks, tubing, etc., often with free shipping.
When buying a regulator, make sure it fits your tank and goes up to at least 15 LPM. Most CHer's find 11 - 12 LPM is a good flow rate.
A rebreather mask is essential when getting a mask. Rebreather masks have a plastic bag hanging from the mask that fills with Oxygen.
Many people like to use bubble humidifier (O2 runs through sterile water before entering the mask) to filter and humidify the O2. They attach in-line, typically at the regulator.