If you have your own compost bin and regularly collect fallen leaves and grass clippings, or you have access to an organic yard-waste collection site, then you have a mulch supply. You might also try one of these as a layer:
• All-cotton rags from old clothing, towels, or sheets
• All-wool carpet or sweater scraps
A word of caution, however: Always avoid debris that could be chemically tainted (public yard waste from open collection sites, synthetic carpet scraps, neighbors' clippings).
If you purchase it, pick mulch that complements your soil acidity or alkalinity (consult the seller or your extension office for help). If you live where heavy wind or hard rains are common, choose heavier and larger pieces that won't fly or wash away as easily. And if termites are an issue, keep mulch away from structures.