Box fans are everywhere but not many of them use 12V DC power. These fans from Miller Tech can even be powered by power tool batteries from Milwaukee, Makita, and DeWalt with special battery docks (sold separately).
Please note these fans are lightweight and inexpensive. Their value lies in the ability to use power from batteries or solar directly, not in a long life or ability to take abuse. They do carry a 1 year warranty from the manufacturer.