First of all, you can get cranks in 170.0mm if you want.
Secondly, Nearly all major brands (Shimano, Sram, Truvativ, etc.) make cranks in a 165mm length.
If you don't want to replace the gears, you're going to have to find another crankset with the same bolt pattern. Keep in mind, there are 4- and 5- bolt chainrings with different bolt circle diameters (ex. 104mm, 110mm, 130mm).
If I were you, I'd go to my local bike shop with your old crankset. Tell them you're looking for a shorter crankset with the same bolt pattern. Ask them if they have anything used that's for sale. If they don't, see ask about buying a new crankset.
Bama