you cant buy ethers my advice is to buy loads a tents then when you get to the underwater aera (SPOILER)where quina trys to swim there is an optinal boss hidden behind a bit of rock called hades if you beat him he will open the ONLY sysnithes shop that can make eithers.

ps hades is hard reccomend being level 55 plus and watch out for his DOOMSDAY attack as it hurts. but he give plenty of warning for a big attack so keep hp high.