If you have neighbors or friends with chickens or other poultry you may be able to score some used bedding/litter. I have chickens and quail and even with three huge compost bins I don't have space for it all so give much of it away when the bins are too full. My problem is getting the browns for the bins so collecting my neighbors' leaves, and getting free sawdust and shavings from local woodworkers helps round out my piles. The chickens help turn the top foot or so of the piles so they earn their keep between that and producing eggs.