i just accidentally found a perfect-fit box for the Onus Order and Event cards (sleeved):
i bought those boxes a month or two ago with the hope that i'd eventually find a use for them. Today i tried it out while reorganizing, and they fit like a glove. Only one is needed - the picture shows two because i have two.
They can be found in Europe here:
(that's where i got them)
But any of the larger hobby shops in the States likely carries similar boxes. Or Amazon.
Maybe other players will post links to other sources (hint, hint!).