You have to go to a medical oxygen supplier, which is not the same thing as a medical supply store. Type "oxygen suppliers" and the place where you live into google. Or look for Lincare, Praxair, Airgas, Linde, Apria . . . those are some common ones. It's pretty common for doctors to fax an O2 prescription directly to a supplier. Does s/he not know any? You could call neurologists, or hospitals, or assisted living places or nursing homes -- they all would have relationships with O2 suppliers.
But if your doctor wrote your prescription for 5lpm, you are probably not going to get the tanks you need, because that is easily satisfied by a concentrator, which is pretty unsatisfactory for CH.
The national suppliers I listed above all have "industrial gas" divisions that will sell you welding O2. Or google welding supplies and the place where you live.
I think I have already linked you to these, but in case . . . .
https://clusterbusters.org/forums/topic/5627-notes-about-welding-o2/
https://clusterbusters.org/forums/topic/6213-basic-non-busting-information/