The official US Topo Quadrangle maps are now online (7.5-minute quadrangle topographic maps), and are updated every 3 years.When updated, the previous online (legacy) quads are then moved into the USGS Historical Topographic Map Collection.
There's a new Fact Sheet, Scanning and Georeferencing Historical USGS quadrangles Fact Sheet 2017-3048 available, https://pubs.usgs.gov/fs/2017/3048/fs20173048.pdf
Scanned images of paper maps up through 2006 are also available for free download from The National Map