Jeff, try the yellow pages under "industrial hydraulic" or similar - large industrial equipment uses up to at least 36" hydraulic cylinders - also, air actuated cylinders get pretty large, and go down to less than 1" diameter. Companies who specialize in repairs of such equipment, if they won't sell you a few, might be able to point you to their source. Also, maybe a logging supply or tractor repair shop.

I haven't tried this, but maybe some of the "pony tail rubber bands" ladies use to do their hair up in buns or pigtails? I've seen some years ago that looked like a mil-spec mini bungee cord... Steve