FindControl returns a control so you need to cast it as a TextBox to be able to get it's Text value.
For example:
TextBox txtId = Repeater1.Items[0].FindControl("id_string");
if (txtID != null)
{
Response.Redirect("Default3.aspx?Name=" + txtID.Text);
}
By the way, I would suggest naming your pages to something more descriptive than Default3.aspx as well as use control names that make more sense, such as txtID rather than id_string.