Try contacting the American Institute of Architects (AIA) at
https://www.aia.org. If you are in the US there is probably a local chapter.
If there is a local school (high school, jr. college, college) try a visit to their bookstore, or an instructor for Drafting.
Also, the local building department may (or may not) be a source for leads.
It has been decades since I was slightly involved in this field, but please realize that there are many hundreds of symbols & standards used in Architecture.
There are symbol groups for detailing Construction (windows, doors, framing, concrete, steel, details for joining beams), Plumbing, Architectural details, Landscaping, Electrical, Appliances, Heating, Ventilating & Air Conditioning (HVAC)... and the list goes on.
The below were found with:
https://www.google.com/search?q=architectural+drawing+symbols+pdf
Here is a start from the U.S. Department of Housing and Urban Developement website:
https://www.huduser.gov/portal/sites/default/files/pdf/Architectural-Drawing-Part-1.pdf
This one may be too basic for you but it includes tips on what to watch out for in drawings:
https://inar.yasar.edu.tr/wp-content/uploads/2015/11/Drawing-Conventions_3.pdf
Here is a symbol group that AutoCAD uses (rather limited):
https://www.g-wlearning.com/cad/7376/resources/data/symbols.pdf
More of an overview of an architectural dwg.
https://www.pearsonhighered.com/assets/samplechapter/0/1/3/2/0132740648.pdf
Cheers,
Tom