How to display total quantity available in stock for a item on TextBox using SQL Server, C# and VB.Net in ASP.Net

I have a form called Customer Order, i want to be able to display the total quantity of a product and display it on textbox called quantity in stock. See html below

<div class="box col-md-6">
    <h3 style="margin-top: 4px">
        <asp:Label ID="Label2" runat="server" Text="" CssClass="glyphicon glyphicon-shopping-cart"></asp:Label>
        Customer Order
    </h3>
    <div class="box-inner homepage-box" style="height: auto">
        <div class="box-header well">
            <h2>
                <i class="glyphicon glyphicon-th"></i>Block Production
            </h2>
            <div class="box-icon">
                <a href="#" class="btn btn-setting btn-round btn-default"><i class="glyphicon glyphicon-cog">
                </i></a><a href="#" class="btn btn-minimize btn-round btn-default"><i class="glyphicon glyphicon-chevron-up">
                </i></a><a href="#" class="btn btn-close btn-round btn-default"><i class="glyphicon glyphicon-remove">
                </i></a>
            </div>
        </div>
        <div class="box-content">
            <div>
                <div id="" class="">
                    <div class=" ">
                        <div class="">
                            <div class=" ">
                                <div class="clearfix">
                                </div>
                                <div id="" class="">
                                    <p class="">
                                        <asp:DropDownList ID="ddlLocation" runat="server" CssClass="btn btn-default dropdown-toggle"
                                            Width="100%" Height="50px" Font-Bold="False" Font-Size="Larger">
                                            <asp:ListItem>Please Select Location </asp:ListItem>
                                            <asp:ListItem>Nkwere Branch </asp:ListItem>
                                            <asp:ListItem>Gabbson Onitsha  </asp:ListItem>
                                        </asp:DropDownList>
                                    </p>
                                </div>
                                <div class="clearfix">
                                </div>
                                <div id="" class="">
                                    <p class="">
                                        <asp:DropDownList ID="ddlInches" runat="server" CssClass="btn btn-default dropdown-toggle"
                                            Width="100%" Font-Bold="False" Font-Size="Larger">
                                            <asp:ListItem>Please Select Item</asp:ListItem>
                                        </asp:DropDownList>
                                    </p>
                                </div>
                                <div class="clearfix">
                                </div>
                                <div id="" class="">
                                    <p class="">
                                        <asp:DropDownList ID="ddlcategory" runat="server" CssClass="btn btn-default dropdown-toggle"
                                            Width="100%" Font-Bold="False" Font-Size="Larger">
                                            <asp:ListItem>Please Select Category </asp:ListItem>
                                        </asp:DropDownList>
                                    </p>
                                </div>
                                <div class="clearfix">
                                </div>
                                <p class="">
                                    <asp:TextBox ID="txtstock" runat="server" CssClass="form-control " placeholder="Quantity in store "
                                        ReadOnly="True"></asp:TextBox>
                                </p>
                                <div class="clearfix">
                                </div>
                                <p class="">
                                    <asp:TextBox ID="txtprice" runat="server" CssClass="form-control " placeholder="Selling Price "></asp:TextBox>
                                </p>
                                <div class="clearfix">
                                </div>
                                <p class=" pull-right">
                                    <asp:LinkButton ID="btnSave" runat="server" CssClass="btn btn-info btn-circle" Text=""
                                        Height="50px" Width="100px" CausesValidation="true" OnClick="btnSave_Click">
                                        <asp:Label ID="Label5" runat="server" Text="" CssClass=" glyphicon glyphicon-cloud-upload"
                                            Font-Size="Large"></asp:Label></asp:LinkButton>
                                    <asp:Button ID="btnReset2" runat="server" CssClass="btn btn-default btn-circle" Text="Reset"
                                        Height="50px" Width="100px" OnClick="btnReset2_Click" />
                                    <asp:Label ID="Label1" runat="server"></asp:Label>
                                </p>
                            </div>
                        </div>
                    </div>
                    <div class="">
                    </div>
                    <!--/#content.col-md-0-->
                    <div class="row" style="width: 100%">
                        <!--/span-->
                    </div>
                    <!--/span-->
                </div>
            </div>
        </div>
    </div>
</div>

Now the store total quantity will be displayed on textbox based on these Inches selected and Categories selected. The inches and Categories are stored in table  called Inches table and Category table.

Both will be displayed and bind on dropdown on Customer order table, when the user selects both the inches and categor it will show the total quantity of that inches and category selected.