Actually that tank at harbor freight is $79
http://www.harborfreight.com/20-cubic-ft-oxygen-cylinder-92810.html.
You really dont need a regulator at all, just open the valve a tiny bit and let it flow to your desired rate. You are not trying to hit a particular pressure, just flow rate and even that need be precise. The one for the disposable ones is just a valve you know.
In realty though, if you are thinking of the refillable tanks. Check with all the welding supply shops in your area. It might be better to just buy a 20cu ft tank filled from them. You buy that new one at harbor freight and you will just swap it for a used one that is full. Most of those places dont actually fill them for you.