Following should work. See I added runat="Server" for txtNotes:
aspx:
<FooterTemplate>
<asp:DropDownList ID="ddlStatusList" runat="server">
</asp:DropDownList>
<input id="txtNotes" runat="server" type="text" placeholder="Notes" />
<asp:Button runat="server" type="button" Text="Add" ID="btnAdd"></asp:Button>
</FooterTemplate>
C#:
protected void btnAdd_Click(object sender, EventArgs e)
{
var txtNotes = (System.Web.UI.HtmlControls.HtmlInputText)(((Button)sender).Parent).FindControl("txtNotes");
var ddlStatusList = (DropDownList)(((Button)sender).Parent).FindControl("ddlStatusList");
}