As of 2016, litter bags for cars are available at Amazon.com and CargoGear.com. Styles and price points of these litter bags vary between different brands and retailers.
Amazon.com carries litter bags for cars that hang over the backseat in square and round shapes and various sizes. This retailers also sells liners for the bags. Many of the bags are made of vinyl and come in a leak-proof style.
CargoGear.com offers litter bags that are made to go on the back of a seat or fit snugly on the floorboard of most cars. Some of these bags offer extra holders for water bottles or other incidentals. This retailer offers a pop-up version that stores when not in use and one that offers tip-resistant properties.