Check around your area to see if anything is available in bulk, if you have a way of hauling it, it's usually cheaper than "sacked"!
I'm able to get "chipper shavings" from an Amish pallet plant free for the hauling about 15 miles from home, mostly Cottonwood. I have room to store it for a year before using it, thus "aging" it for a year. This makes great mulch for flower & vegetable gardens, as well as tees & shrubs. I don't, however use it next to buildings, as it could promote termites.
Other sources might be a sawmill or City brush waste. ~~ grnspot110