Hey i havn't been working alot in VB but im very into C# and i know that you in C# can get a char from a string simply by doing this:
char c = stringname[index];
for larger numbers you can use a while loop and check if that char is a digit like this:
int i = 0;
string numberindex = null;
while (char.IsDigit(combobox.SelectedText[i]))
{
numberindex += combobox.SelectedText[i];
i++;
}
int result = int.Parse(numberindex);
the above should work with a few modifications :)
hope this help