Once you have obatined the Sans Culpra Scroll, travel to Sans Culpra and you will find a message in a bottle. The message contains a password to get into Earl's house in Waterport. Password is "money". Travel to Waterport and go to Earl's house. You will get an error advising that Earl is at the Casino and to come back in 24 hours. After 24 hours, go back to Earl's house and you will be able to buy oxygen tanks.