Sorry for previous answer, I created a function for you, try this, You need to pass your string, character to find and occurance number as parameters. Here is a demo.
public static int getSpecifiedIndexOf(string str,char ch,int index)
{
int i = 0,o=1;
while ((i = str.IndexOf(ch, i)) != -1)
{
if(o==index)return i;
o++;
i++;
}
return 0;
}