I have been using the following URL for the past 3 years without issue. However, it has stopped returning results.
URL: https://query.yahooapis.com/v1/public/yql?q=s
Yahoo is aware of this issue and their engineers are working on it:
ref: https://forums.yahoo.net/t5/Yahoo-Finance-help/http-download-finance-yahoo-com-d-quotes-csv-s-GOOG-amp-f/td-p/387096