The maps are generated directly from elevation data--all you need is an array of elevation values and the full maps are generated from those. A good source of elevation data for the USA is the National Map [1], but a much more comprehensive source is the SRTM Space Shuttle data [2] (it covers almost the entire earth at a 30m resolution).

[1] https://viewer.nationalmap.gov/basic/ [2] https://dwtkns.com/srtm30m/