ASP.NET

<asp:Repeater runat="server" ID="Repeater1" DataSourceID="dsLocations">
    <HeaderTemplate>
        Items<br />
    </HeaderTemplate>
    <ItemTemplate>
        <b><%# DataBinder.Eval(Container.DataItem, "City") %></b> <b><%# DataBinder.Eval(Container.DataItem, "State") %></b>
        <asp:HiddenField ID="HiddenField1" runat="server" Value='<%# DataBinder.Eval(Container.DataItem,"LocationID") %>' />
        <asp:Repeater ID="Repeater2" runat="server" DataSourceID="dsUserData">
            <ItemTemplate>
                <p>
                    <%# DataBinder.Eval(Container.DataItem, "LastName") %>,
                    <%# DataBinder.Eval(Container.DataItem, "FirstName") %></br>
                </p>
            </ItemTemplate>
        </asp:Repeater>
        <asp:SqlDataSource ID="dsUserData" runat="server"
            ConnectionString="server=.\SQLEXPRESS;Initial Catalog=LinqTest;Integrated Security=true"
            SelectCommand="SELECT * FROM UserData Where LocationID = @LocationID"
            OnSelecting="dsUserData_Selecting">
            <SelectParameters>
                <asp:Parameter Name="LocationID" Type="Int32" />
            </SelectParameters>
        </asp:SqlDataSource>
    </ItemTemplate>
    <SeparatorTemplate>
        <br />
        <br />
        <br />
    </SeparatorTemplate>
</asp:Repeater>
<asp:SqlDataSource ID="dsLocations" runat="server" ConnectionString="server=.\SQLEXPRESS;Initial Catalog=LinqTest;Integrated Security=true" SelectCommand="SELECT * FROM Location"></asp:SqlDataSource>

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;

public partial class _Default : System.Web.UI.Page 
{
    protected void Page_Load(object sender, EventArgs e)
    {
    }

    protected void dsUserData_Selecting(object sender, SqlDataSourceSelectingEventArgs e)
    {
        HiddenField hf = Repeater1.Controls[1].FindControl("HiddenField1") as HiddenField;
        if (hf == null)
            return;

        e.Command.Parameters[0].Value = hf.Value;
    }
}