I've had a German army poncho for some time now, very sturdy, rubbery plastic thing. Great for setting up a shelter, totally waterproof as well. Will keep you dry even in a bad storm though perspiration is a big problem. The US poncho is also pretty good. Just about any other military grade poncho along the same lines will do the job.

Like Chris, I see no need for a poncho liner. Little practical value and too bulky. Better carry a sleeping bag instead. Anyway, a $20 surplus poncho is all I need. There may be "better" ponchos or tarps out there but I just don't care, the military stuff is inexpensive and works just fine.