Came across this upper/lower receiver on ar15.com
I thought it looked pretty neat. Nothing I would want to hunt with but could make a pretty cool build with.
http://core15rifles.com/component/virtuemart/?page=shop.product_details&flypage=flypage.tpl&product_id=173&category_id=14&vmcchk=1
you can also order just the lower or the upper.