A few random thoughts....not recommending any of this....
Not the most pleasant thing perhaps, but depending on where you live and what the local wildlife and road conditions are, would road-kill be viable? Around here I could gather enough for a whole pack of dogs all year long. Little to no cash input (gas?) and very little time involved. I'd check the laws first and probably plan to cook anything gathered to be on the safe side.
Another source might be any commercial chicken/turkey/hog farms near by. They all have animals die every day that have to be disposed of (bury, compost, burn), maybe you could collect them for the dogs. A lot of them have reduced/eliminated the hormones and meds of the past, but I'd still ask if going this route.
