Well - it's a question what you want to do with it.
Do you need a controller with pads (to use as a simplified drum surface), or do you need a mixer controller.
For a pad controller Akai has a nice range. I have used the LPD8 and later the MPD18 with succes in X1
M-audio has some nice cheap ones, but they are rather limited (well - the cheap ones are certainly).
If you are on budget and want motorized faders in my opinion the Behringer BCF2000 is unbeatable. It has even a special Sonar modus, and works without a problem with X1.
If you do not have much room for a full motorized fader surface you can opt for the Presonus faderport or the Frontier AlphaTrack (the last one I did not thest under X1, so I cannot say anything about that.
If you want something that is very usable with the X1 Matrix, you can take a look at the Novation launchpad.
So - maybe it is better you give some indications what type of surface you want and what budget you have.