Call the social services dept at your local hospital and ask them where you can go for help. Ask the doctor who told you to use it where you can get it cheaply since you have no funds. Ask your family for assistance. Ask who the oxygen suppliers are for your area...ask your doctor as I'm thinking he would know...call them and find out exactly how much it would cost and whether they have any assistance programs. If you have a charge account, put it on your card and pay the minimum payment it calls for. Ask your doctor if an inhaler would help you until you,re on Medicare.
These are hits for other questions asking the same issue, i.e., how to pay for oxygen. You might find some helpful information from these threads:
https://www.agingcare.com/search.aspx?searchterm=paying+for+oxygen
https://www.medicare.gov/coverage/oxygen-equipment-and-accessories.html