Are you flailing around without a product owner? I have the simple cure... appoint yourself. Not because you want to be the product owner, but because everyone who really wants to be the product owner will come out of the woodwork!
In just declaring what the priorities are and watching other people's 'beloved' items fall below the iceberg water line sprint after sprint... you'll get some help in prioritizing. You may even find your strongest candidate for a real product owner!
Happy Sprinting Agilistas!