I would try an match the same model and speed if possible as that will give you the best probability that it will work. But you may not be able to find the same model either.
Mixing and matching RAM does not always work, this is why RAM is sold in kits as they are tested to work together. If you get the 2400mhz stick and the system boots, it will only run at 2133mhz because that is the lowest speed of the sticks.
Chances are, it will work, but I would get the second stick of RAM that accepts returns just in case.