What you want to do is called getting a localized string representation of a number.
With your example, that would give:
using System.Globalization;
CultureInfo usCulture = CultureInfo.CreateSpecificCulture("en-US");
int intValue = 1241202;
double realValue = 1241202.2;
string intValueRepresentation = intValue.ToString(usCulture);
string realValueRepresentation = realValue.ToString(usCulture);