You can use either medical or industrial oxygen. There is actually no difference in the quality. Medical (99.8% clean) and industrial (99.5% clean) Oxygen are both filled from the same container.
Only the tank is different. Medical is aluminium while industrial is made from steel. If you get an industrial container look for a newish bottle to avoid corrosion inside the tank.
If you have a prescription you can order a 20 L medial oxygen bottle from your local oxygen supply.
If you have no prescription do not worry. Just get a 20L bottle from any local welding gas supply.
We understand that finding a place that offers industrial and medical oxygen can be difficult, so we have done the work for you! Here is a list of suppliers and their contact websites. Of course, there are many more suppliers, some which may be more local to your area but hopefully this can be a starting point.
AGP – American Gas Products
(including North America, South America, Europe, Middle East and Asia)
Air Liquide Creative Oxygen
Australia & New Zealand
BUNNINGS Building Supply
Mitre 10 and many more.