Use the AutoCompleteExtender Ajax control by mapping to your relevant text box
using webservice method as listed below e.g
<asp:TextBox ID="txtCustomerName" runat="server" class="txtboxtxt"
onblur="FocusChValue();" onfocus="DoFocus(this);" TabIndex="0" Width="282px"></asp:TextBox>
<cc2:AutoCompleteExtender ID="AutoCompleteExtender2" runat="server"
CompletionInterval="500" CompletionSetCount="12" EnableCaching="true"
Enabled="True" MinimumPrefixLength="1" OnClientItemSelected="fillCustomerCode"
ServiceMethod="GetCustomerNameCompList" ServicePath="wsSales.asmx"
TargetControlID="txtCustomerName">
</cc2:AutoCompleteExtender>