How to rig a rain tarp without trees -- poles, maybe??

Try this site Government Liquidation. It is an auction site for surplus military items, including netting and tent poles. Most lots will come with both netting and poles but occasionally you will find just the poles, pole top spreaders, and accessories (stakes, rope, etc.) The camo systems are designed for use with or without trees.

BTW this particular lot is at Ft. Lewis for pick up.