The baby turtles, once so common, are currently sold only illegally on the streets of Chinatown in New York or out of the back of a pickup truck in Florida. You can buy a 4" turtle, however, at most pet shops. With proper cleaning, a turtle that size can be kept in a 20 - 30 gallon aquarium. Filtration helps, but you must buy a specialized model for reptile tanks or a very expensive outside cannister filter that you set on the floor below water level. Even so, the tank requires partial water changes. If aesthetics aren't an issue, you can house the turtle very well in a large plastic storage container that is munch lighter to move around and less vulnerable to breakakge (and cheaper). Be sure to have a clip-on incandescent light fixture for basking and a place to get out of the water. You don't necessarily need elaboraate decoration or gravel in the tank. Although a large female red-ear can reach 10 inches, males are smaller and growth may not be as rapid as predicted.