to be honest if you go to a supermarket you can get a plastic (tupperware) tub like a big jug with lid for keeping cereal in (just like ion said above)
i got a fairly large one and it holds a bag of paint at a time and the spout/hole is plenty big enough for the paint to come out
and there only a few quid
but if you want a caddy if you go to group buys on here someone was thinking of doing one
Put all your paint in a empty box and scoop it into your pots.
One good idea I seen down at the masters was what looked like a old plastic watering can with nozzle cut right down, seemed to work a treat.