You might check with the city to see if they have mulch for free. Sometimes the Botanical Garden has extra mulch as well, though probably not free.

Another thing you can try is picking up bagged leaves that people sometimes put out for trash pickup. Spread the leaves out on your grass and run over them a couple times with the lawnmower. Then spread them on your garden beds and water them to weigh them down. They make great mulch and are really good for the soil too.