Use you existing filing cabinets and pick them up and put them on moving, rolling bases to allow the front level to slide to the left or right to access the layer of cabinets behind the front level.
If you have existing filing cabinets lined up down a wall, then you could add another layer in front of your existing row of filing cabinets. This is a real space saver and allows you to store more in the same floor space. The bonus is that you get to reuse your existing filing cabinets. The concept can be seen below in the moving animation. The units in the front slide left and right to access the level behind. Think of the recycle content if you are dealing with a LEED project. You can reuse the existing file cabinets and put them on the moving bases. This is a big cost savings.