Googling for typeahead  dropdown ajax returns quite a lot of references:
http://www.google.com/search?sourceid=navclient&aq=t&ie=UTF-8&rls=GGLG,GGLG:2005
-46,GGLG:en&q=typeahead++dropdown+ajax

For the loading part, you should filter the items on your datasource before
databinding.

Hello Peter,

So, what's your expectations from the control? What do u wanna get?
If not extract all records then u need change the usability of your contol.
I'd recommend to use AutoComplete dropdown list, where the selection creteria
is based on the first 2 symbols which user typed

---
WBR,
Michael  Nemtsev [.NET/C# MVP] :: blog: http://spaces.live.com/laflour 

"The greatest danger for most of us is not that our aim is too high and we
miss it, but that it is too low and we reach it" (c) Michelangelo

P> Does anyone know of OpenSource .NET 2.0 ajax dropdown box with
P> typeahead feature and ability to load only X amount of items?  I have
P> over 20,000 names in a database table which I would like to allow
P> user to select from a dropdown box, but I don't want to load all of
P> them at one time.
P>
P> Thank You
P>
P> Peter
P>