If I were you, I would do this:
- Buy a test kit for the water and test everything daily, pH, ammonia, nitrite and nitrate
- Change about 20% of the water whenever the test results aren't good
- Feed a small, small amount of food at most once or twice a day
- Look for a 10 gallon (20 gallon or more is even better!) to keep the fish in
- Save my money to buy a good filter for the 10/20 gallon instead of looking for a waste of money in the form of a battery-run filter
This way your fish should be able to survive until you'll have the tank
Like I mentioned before, if you won't get a tank, don't keep the fish.