Try the hardware stores: Aco (if you have them),Home depot, Lowes... They are not screw on tops, but they can range from snap on tops that are not water tight to snap on tops with gaskets. Paint stores can also possibly supply you with new buckets not used with paint.
I think the I/O buckets are crap. They changed design a few years back. They are smaller and they leak! The old ones were 6 gallons and had nice lids, well made. Oh well...
http://www.usplastics.com has lots of different buckets, but they are more expensive than local sources.
Oh they still make them I just haven't found a source yet! I called the Lectica (sp?) company direct and the minimum order was something like 50 buckets! So then you are left with finding someone they sell them to. The usplastics site has the same brand, they are just a little shorter (more like 4 gallons).