If you have a plot on a council run allotment, try contacting them. They often have wood chips to get rid of from their maintenance activities and will deliver a trailer load to your allotment for nothing.
Also don't waste the old chips when they start to break down after a few years. If you have put a membrance down, then carefully rake off the big chips on the top, and you will find a wonderful barky compost underneath that can be dug straight into the ground, or used as a mulch.