I usually see them hanging on display in the water asile. Last time I was at babies R us they had several - some fit juice bottles others fit water bottles.
If you have a prefered brand you could just buy a few of the sport size and then re-use the tops on other sizes. I prefer this because they usually fit a little better.
WDW sells Disani so I usually stock up on several Disani tops prior to a trip.
TJ