If you Google "SJS Cycles" and use their search faclility, you will come up with any number of shorter cranks. The thing is, are you sure you are approaching your problem from the correct angle? Unless you are fairly short in stature and hence riding a smaller frame (generally smaller frames have shorter cranks fitted) I'm not sure shorter cranks are the answer.
Do you use clipless pedals? If so then knee pain can be caused by cleat misalignment. Muscular-skeletal quirks can also cause it, for instance if your foot is not exactly flat on the pedal platform.
I'm not saying shorter cranks won't help, but you would be better advised to seek the help of good bike fitter first.
If you let us know where you are based, someone will point you in the direction of a good man.