Hello, Grammarcrazed, and happy New Year.
I agree with you that we have always been taught (as well as teach our students) that "could" is used for general past ability while "was/were able to" is preferred to express past ability + actual performance.
In Betty Azar and Stacy Hagen's Understanding and Using English Grammar, Fifth Edition, we can read in table 10.3 Expressing Past Ability (page 191) the following (I have combined the explanation and the examples which appear in different columns of the table so as to make it clearer in this non-tabular format):
For a single action in the past affirmative, was/were able to or the simple past is used, as in (e) Maya was able to do well on her exam (or Maya did well on her exam), INCORRECT: Last week, Maya could do well on her exam. Could is not typically used*. For the negative, both verbs are possible: Maya couln't do well on the test / Maya wasn't able to do well on the test.
*Exception: Could can be used in the past for one action with these sense verbs: hear, feel, see, smell, taste; and the verbs understand, remember, guess.
I know that "find" is not among the verbs above, but please notice that Azar & Hagen say that "could" is not typically used to refer to a single action in the past. My impression, based not only on the books I have read but on my actual experience with the language, is that "could" can occasionally be found with some verbs, especially when not much effort is involved. I would definitely use was/were able to in sentences like the one about Maya passing her exam above or in a sentence like this one:
- When I was young, I was a professional climber. I'll never forget when I was able to reach the top of the Everest.
I went and searched in the Corpus of contemporary American English and I could find it in academic prose...
In your sentence above, I find that there is not much effort involved in the finding in question and also that "find" is quite close to "see" or "come across," don't you think?