You don't need to choose just 1 substance. If you can use a variety, it will add more benefit to the soil more quickly. Any organic matter that reasonably doesn't have seeds in it can be added to the "mulch." Do people bag leaves where you are? You can use any organic matter that is easily composted, such as leaves, shredded wood, kitchen scraps - especially coffee grounds, small trimmings from shrubs or trees, shredded paper, sawdust, whatever is available. Sometimes tree trimming companies love to have somewhere to dump a load of wood chips. You could call around and ask. The things I avoid using in compost & as mulch are bigger pieces of wood, thorns, meat & bones, anything salty.