If you have a particular architect in mind, try a subject or keyword search for the architect's name and "notebooks, sketchbooks, etc." For example:
If you know the time period you're interested in, but not the particular architect, you could do a search for architectural drawings and sketchbooks from that time period, such as:
For an even broader search, simply try searching for architecture and notebooks or sketchbooks. For example:
David Adjaye and Phil Freelon, Sketch for National Museum of African American History and Culture
Archigram Paper One (1961).
"Drawing is an exercise of transforming space into a habitable place. I use Post-its as graphic aphorisms, collecting the observations and solutions reached in the rehabilitation of public spaces... The Post-its are both process and result: they are non-linear and thematic. They are a way of looking at reality in order to transform it"
Image and quote from AR Editors, "The Open Sketchbook: Collecting Ideas," Architectural Review, July 7 2020,