What TSA wants is a bag that is the approximate dimension of a quart-size bag and is clear. That means that a ziploc bag from the grocery store works just fine as long as it is not larger than one quart.
It can be as simple as a clear plastic sandwich bag. If you plan to carry liquids on a plane, they do need to be in a clear plastic bag that is quart size at it's biggest.
You are allowed to bring a quart-sized bag of liquids, aerosols, gels, creams and pastes in your carry-on bag and through the checkpoint. These are limited to travel-sized containers that are 3.4 ounces (100 milliliters) or less per item.