look into ajax autocomplete
Jumptree ASP.NET 2.0 Project Management - With Source Code, Free to Non Profit Org and Effectively Manage Projects/Tasks/Milestones
You definitely want to use Ajax (as was suggested). Since you're using ASP.NET 2.0, your best bet is to look into the ASP.NET AJAX framework. For complete information on the ASP.NET AJAX framework, visit http://ajax.asp.net. Specifically, to accomplish your scenario you may want to use the AutoCompleteExtender contained in the ASP.NET AJAX AjaxControlToolkit. The AutoComplete extender enables you to easily search a database as users enter text into a textbox.
Let me know if that helps~