Seattle fabrics has cargo net on this page:
http://www.seattlefabrics.com/mesh.html
5000 Cargo Net
Wide nylon. Large 1" or 2" holes.
Uses: Sport screens, gear lofts, storage bags, etc.
Width: 6'-12'
Colors: Call us at (206)535-0670 or email us for colors, widths and prices
$10.00-$14.00/yd
You would need edge banding tape and bungees but to outfit a whole trailer it would save a lot of money over the pre-made ones. Add a lot of work though!