I'd get yourself one of these:
My favourites are painting knives shaped something like 26 or 27 in the pic, but play with various profiles if possible to find what suits your hand best. The blades on various makes vary in stiffness and you'll have to try for yourself to see what you prefer but I would recommend for light use getting one that's more flexible rather than stiffer, if there's a choice.
Aim to pay no more than about a fiver; they're a good investment as a well-made knife should last your whole life with luck. Go for stainless steel, chromium or nickel plated as acrylic/vinyl paints quickly tarnish carbon steel.
In addition to their usefulness for dipping and transferring paint from open-topped containers I would recommend getting used to mixing paint with one - it feels a little odd to begin with but once you get used to it you'll wonder how you did without. Big step up from using a brush, brush handle or some random stick especially for mixing stiffer paints, larger volumes of paint as well as your favourite custom muddy mix. They can also be pressed into service as a miniature trowel.
The standard flat palette knife I would not recommend, without going into detail they don't really work as well overall and the flat shape means you're more likely to get your knuckles in the paint. The dogleg type are worth considering (4 or 5 in the photo) but I prefer the finer tip on a painting knife for dealing with smaller quantities like we tend to be dealing with.
Einion