AJAX :: Asynchronous Data Load In Dropdownlist?

first i'll describe my problem:

I have payment form with a clients dropdownlist that is populated from a database on form load with:

value field = clientIdtext field = nameId
<asp:DropDownList ID="selClients" runat="server">
</asp:DropDownList>

When user makes a payment, it's necessary to chose a client from selClients, and so i get the clientsId value and build my insert SQL query.

Until here, no problems.

The problem with my UI has started since i had a lot of clients been registered: My dropdownlist became huge and it's taking too much time to load it's contents on page load. Also the dropdownlist area is taking a large area from page screen.

So i tryied a solution with jquery autocomplete plug in. I put a textbox with autocomplete for client name and another hiddenfield to receive the clientId when user select a item.

That solves both problems:

No more large amout of data loaded on page loadThe dropdownlist doesn't take a large area from page screen
But it gave me another problem!

With a textbox, there is a possibility for wrong input client name. So if i don't have a valid clientId in hiddenfield, it will generate a database INSERT exception because it doesn't respect the client table constraint:

payment table REFERS client table

I'm trying to find kind of dropdownlist where the user can type text with autocomplete. Like this, the user will be able to select only data within the possibilities of the dropdownlist and i always will have a valid clientId.