There's a few things you can do to try and ensure you always have a decent selection of notes.
Firstly, when you need to draw, say 20,000bt, choose "enter amount" and enter 19,900 instead. That gets you 19 * 1000 bt notes, 1 * 500 and 4 * 100, which is a start.
Secondly, as others have said, whenever you buy something at 7/11, Family Mart or Tesco pay with a large note; they're used to people doing this and don't get annoyed by it, even if you're only buying a 5bt bottle of water. Of course, this does usually involve you having to wait around for about half a week while they go to the change tin as they never seem to have enough change in the actual till itself (this can also happen when you pay 10bt of a 5by purchase, to be fair...)
When paying for a meal, again, use higher denominations of notes.
If you still have trouble, then you'll probably want to walk in a bank and ask
![Smile :)](./images/smilies/icon_smile.gif)