Your question isn't very clear, but I think I've worked out what you mean...
Try this:
bool condition = true;
string basePath = @"D:\Temp\";
string fullpath = "";
int i = 1;
while (condition)
{
fullpath = Path.Combine(basePath, i++.ToString("000000"));
condition = Directory.Exists(fullpath);
}
Directory.CreateDirectory(fullpath);