50 lb feed sacks from the farm supply work great. The feed store near me sells them for 50 cents each, and the guys who work there were happy to save the plastic used to wrap pallets of fertilzer and such. A week's worth of plastic filled up five bags. Use a plastic zip tie to close the bags. Like tmcnamara7 said, just use a marker to draw whatever target you want on the bag. I traced the vital from a 3D deer target and made a template to use with my bags. Works best if you draw your target before stuffing the bags.
If you don't like white, I've found the bags take spray paint pretty well. Glossy finish seems to hold on longer than flat. I've painted several up to look like the side of a deer. When a bag starts to wear out, just stuff the whole thing into a new bag.