Where might one look to find examples of such code? I've never found algol outside of wikipedia

https://rosettacode.org/wiki/Category:ALGOL_68

https://github.com/search?q=algol68&type=repositories

Without knowing what your interests/motivations and backgrounds are, it is hard to make good recommendations, but if you didn't know about rosettacode or github I figured I should start with that

Old papers and computer manuals from the 1960's.

Many have been digitalized throughout the years across Bitsavers, ACM/SIGPLAN, IEEE, or university departments.

Also heavily influenced languages like ESPOL, NEWP, PL/I and its variants.