Trail mulch if you can find it. I get it from my county compost site for $10 a yard and it consists of much larger wood chips and sticks than will be found at a supply center. The county also uses this on their walking trails through nature preserves, so your application is exactly what these chipped are used for. Because of the sizes trail mulch is less prone to washing away, but It isn't pretty, just functional and economical.

I also get wood chips deliver through a place called chipdrop for free. I get more than I want because you get a truck load, or loads, so check to see if chipdrop works in your area and request an order of what you want. My neighbor and I received over 20 cords of free hard wood logs delivered through chipdrop for firewood two years along and about 25 yards of chips.

Another free source are tree cutting companies. Last fall I asked a tree company if they wanted to dump there chips on my land and they jumped on it and ended up with 4 truck loads. None of these chip sources are finely shredded chips you'd buy in bags at the store, and not the best for nice landscaping but for walking paths they rock.

The last free or low cost option is repurposed decking boards found free on craigslist or where ever. Cut the deck boards into the width of your paths, drill two holes on each side and weave a rot and UV resistant rope through the holes. They can also be rolled up, moved, and finally trashed when they rot out.

The last not so great option is old used artificial Turf. It's not so great because I rarely see it for free, but usually can find it for sale for more than I'm willing to pay.