If you can't afford a bigger tank, you could get one of those 20 or 30 gallon Rubbermaid tubs. You will need to make some kind of raised platform so the turtles can climb out of the water and dry off. Make sure your basking light is aimed over this basking area. Also make sure you get one of those spiral bulbs that provides UVB and put that next to the regular light bulb that you use for heat/light. The next thing you will need to buy is a filter to keep the water clean. If the air temperature in your house is below 80 degrees, you will need to buy an aquarium heater to keep those babies warm. Otherwise, they will get sick.
Keep asking questions and take a look at the really good articles on this website (
www.redearslider.com) to learn more about what your turtles will need to stay healthy and happy. You will need to plan to spend some more money to get the filter, lights and a water heater.