Welder here. I don't buy helium for welding any more because it's too expensive; most welders use argon nowadays. But in any case the whole point of using inert gases in welding is to keep oxygen away from the hot metal. If welding-grade helium tanks contained any significant amount of oxygen they would defeat the whole purpose. And yes, for filling balloons, you just buy a tank of welding helium and screw a special balloon regulator onto the outlet port. The gas is identical. Makes you talk in a squeaky-high voice if you breathe a little of it; kills you dead if you breathe a lot of it.

(Edit: I know somebody is going to ask why it kills you. Helium is not toxic, but it suffocates you if you breathe a lot of it without a break for -- you know -- real air.)