So I did finally find Alexander in the 1850 census of New Rochelle - thanks to all you insistent folks :-)The indices were of course useless.I simply had to sit down and go through the census page by page recording each Flandreau.Eventually I reached the families of Alexander and his brother (or cousin?) Irving.
1850 census for New Rochelle, Westchester County, New York page 165 [p328]:
Dwelling 64, Family 77
Alexander Flandreau, 30, male, carriage maker, born in New York
Elizabeth Flandreau, 21, female, born in New York
Mary A. Thompson, 22, female, born in England
James Flandreau, 3, male, born in New York
Alexander Flandreau, 1, male, born in New York
Dwelling 64, Family 78
Irving Flandreau, 39, male, Cooper, born in New York
Levinia Flandreau, 36, female, born in New York
Eugene B. Flandreau, 17 male, Boatman, born in New York
Eliza Flandreau, 10 female, born in New York, attended school that year
James Flandreau, 8, male, born in New York, attended school that year
Rachel A. Flandreau, 6, female, born in New York
Alexander G. Flandreau may be the young Alexander mentioned in this census.
The dual page numbers are the printed number and the number written in the census pages.
As their families share the same house, Alexander and Irving are likely to be brothers.Since both families have sons named James, it's possible that James may have been the name of the father of Alexander and Irving.The 1830 census has at least two James Flandreaus who had sons of about the right age to be Alexander (then 9 or 10) and Irving (then 18 or 19).
I've lost track of a message in a newsgroup somewhere in which someone told me they had seen the will of the elder Alexander Flandreau.If anyone has access to this information, I would be very interested indeed.