For your requirement just replace the '+' with any other character. Check the sample
Response.Redirect("search_donor.aspx?bg=" + DropDownList2.SelectedItem.Text.Replace('+', '/') + "&st=" + DropDownList3.SelectedItem.Text + "&city=" + DropDownList1.SelectedItem.Text);
string str=Request.QueryString["bg"].ToString().Replace('/','+');