Yeah do like 24 said and go to a welding supply shop and tell them you want to buy a smallest tank. You don't want the bigger ones because they weigh too much. You can just play stupid and say it's for your husband and you think he welds. Don't ask them to fill it for you because they can't. They buy them full already. And the difference between medical oxygen and welding oxygen is only about .2% so don't even try to get the medical oxygen.
It costs about $60-$70 for the first tank and then about $20 after that for full ones. Make sure to totally empty the tank before turning it back in.
You have the choice of either using a concentrator or the tanks. Concentrators cost about $500-$600? But then that's all your cost till it breaks down. I told you the price for the tanks. A full small tank last about 1-3 months depending on how much you use it.
But don't worry about it or look like they should sell it to you. Go right in there and tell them you want a small tank for your husband. And get irritated if they start asking questions. All you know is your husband wants you to get one.
And make sure to have some place in your car where it can semi-safely be placed where it isn't going to roll around if you can. You don't want to hit the brakes and have it roll and bust off the valve on the top. Not Good! And no open flames. Now go get it! :-)