<script>
    function isNumberKey(evt) {

        var charCode = (evt.which) ? evt.which : event.keyCode
        if (charCode > 31 && (charCode < 48 || charCode > 57)) {
            alert("Please Enter Only Numeric Value:");
            return false;
        }
        return true;
    }

    $(document).ready(function () {


        $('#TextBox2').keydown(isNumberKey);

        $('#TextBox3').keydown(isNumberKey);
    });

</script>

Select Category:

    <asp:DropDownList ID="DropDownList1" runat="server" DataSourceID="SqlDataSource1" DataTextField="Catname" DataValueField="catId" AutoPostBack="True">
    </asp:DropDownList>
  Name:

    <asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>
            <asp:RequiredFieldValidator ErrorMessage="Enter Name" ControlToValidate="TextBox1" runat="server" ForeColor="Red"></asp:RequiredFieldValidator>

    Price :<asp:TextBox ID="TextBox2" runat="server" ></asp:TextBox>
                            <asp:RequiredFieldValidator ErrorMessage="Enter Price" ControlToValidate="TextBox2" runat="server" ForeColor="Red"></asp:RequiredFieldValidator>

    Quantity
    <asp:TextBox ID="TextBox3" runat="server"  ></asp:TextBox>
                  <asp:RequiredFieldValidator ErrorMessage="Enter Quantity" ControlToValidate="TextBox3" runat="server" ForeColor="Red"></asp:RequiredFieldValidator>