Well I don't know how much information you have, but I was able to find a 1900 census record for an Isaac M. Alexander.
I believe this is your Isaac because he's married to a Willie Alexander (even though the census record says he's his 24 year old daughter - when he's just 34 himself).
He married Willie in 1896 in Franklin County, Louisiana.
The 1900 census shows him and Willie still living in Franklin county and their children as Marion, Isaac C., and Frank F. Alexander.
There are 3 other children who are Isaac's children, but most likely not Willie's.These children are John R., Daniel M., and Marvel Alexander.
I don't know where Isaac M. Alexander ended up in 1910 because Willie is re-married at that time.
The 1900 census did state that Isaac was born around 1866 in Mississippi, so I looked for earlier census records for him.
In the 1880 census, there's an Isaac Alexander living in Tensas, Louisiana.His record says he was born about 1866 and that he was also born in Mississippi.This may be him.He lives with his father John F. Alexander, mother Emma, and sister Melinda.
The occupation for 14-year-old Isaac says he works on a farm (his father, John is a farmer and Emma keeps house).
In the same 1880 census, John F. Alexander's record shows him as being born in Mississippi around 1843 and that his parents were also born in Mississippi.Emma was born in Louisiana around 1850.Isaac's sister Melinda was born about 1873 (also in Mississippi).Sometime between 1873 and 1880, the family moved to Louisiana.
That's about all I could find.Hopefully its the Isaac you're looking for.Good luck!