Open the PDF to Chapter 1, find "Selected Astronomical Constants." Copy these into a JSON file or a Python dictionary. The Supplement gives you the exact definitions (e.g., the defining constant for the astronomical unit is exactly 149,597,870.7 km).
If you cannot access the PDF, these are legally free or equivalent resources: the explanatory supplement to the astronomical almanac pdf
If you want a legal, clean, searchable Explanatory Supplement to the Astronomical Almanac PDF, here are your best bets: Open the PDF to Chapter 1, find "Selected
When using the PDF, cite as:
Urban, S. E., & Seidelmann, P. K. (Eds.). (2013). Explanatory Supplement to the Astronomical Almanac (3rd ed.). University Science Books. [PDF, available from your library or publisher]. Urban, S
If using a library-provided PDF, add the DOI or URL of the institutional access point.