OK guys, you asked for it

:
1. If you want to be able to use the sub without disturbing your neighbors overly much, you have to improve your airborne sound isolation. In your case, this requires you to create a so called double wall. Building a new wall well on the inside of the older wall will be quite efficient - if you do it right. The longer the distance between the new and original wall and the higher the mass of the new wall, the higher the sound reduction will be. You may be able to get an additional 10-15 dB of LF reduction this way. Just staggering the studs in the original wall will not do much to improve LF sound isolation but will make MF and HF isolation significantly better.
2. Reducing structure-borne sound may also be a good idea after taking care of the airborne isolation. The ONLY way of doing this efficiently is mount the sub elastically according to my previous post. Spikes will not lead to any useful level of LF vibration isolation. The reason for recommending corner placement is that the mechanical impedance of the structure Zstruct is higher in the corners. This makes the ratio of Zstruct/Zlayer higher and thus the isolation more efficient. Increasing the sub's mass will further improve isolation.
3. A way of testing if the material under load is still linearly elastic is to first load it with say half the sub mass and noting the deflection. If the material deflects twice (or close to twice as much) as much when loaded with the sub, you are home free.
4. What you need to achieve with your elastic mounting of the sub is a resonance frequency at least an octave below the lowest frequency you are going to reproduce. Depending somewhat on the dynamic stiffness of the material you are using and the mechanical impedance of the floor, the recommended 1/4" will probably be enough to make this happen. A deflection significantly lower than 1/4" will make the resonance frequency too high and may in fact increase the floor vibration levels if you are unlucky!
Note that a stiffer material may be used if you just make it thick enough and/or with an area that is small enough.
Another positive side effect from the vibration isolation is that the sound of the sub will be improved, sometimes dramatically so as you will not be listening to the SQ of your floor boards

anymore.
5. Since I am not based in the U.S., I am unfortunately not able to recommend a source for this stuff. Perhaps some of the other forum members are able to help you here. Remember, you will not need any fancy high-loss space-age material to do this, plain old foam or rubber will do nicely.