One place to look is the county site under usgenweb. Start at www.usgenweb.org
, click on 'the project's state pages', then 'map of US' then NY state. Next click on 'county selection page' then use the drop down list of counties to select St. Lawrence county. The 1870 census is transcribed, indexed and online there courtesy of Anne Cady. Also, extensive cemetery, newspaper, voter and church records.
The 1880 census is online for the entire nation (and 1881 for Canada) at www.familysearch.org
. Maybe that will get you started.
I believe there are Larocks in the county to this day. At the usgenweb site, click on 'places'. Make a note of the town names. You can use www.411.com
and look up any Larock in any town if they have a listed phone number. If you don't mind calling or sending notes to strangers, it could lead to something.
Good luck -