What should I use between spikes and wood floor?

I agree with Slappy. My neighbor was putting in new countertops and I grabbed the chunks of finished one-inch pieces of marble from the debris pile. Works great.

I realize that this is not an answer to your question, but I wonder why you (or anyone) wants to couple speakers to a (suspended) wood floor. Personally, I want to hear my speakers not my floors, and I isolate them from the floors for whatever good that really does, which I assume has something to do with the frequencies at which the speaker resonates and whether or not these resonances would excite the resonance modes of the floor. Where have I gone wrong (or have I)?

I used dimes on my hard wood floors, the whole project cost me .80 USD. You can also use pennies if the budget is real tight, or quarters if you want a higher level of performance. All joking aside though, I got this reccomendation from Sumiko audio, and it has worked well for me.

I use short spent pistol shell casings.Plenty sizes shapes and use softer alloys in aluminum or brass. Polish up very nice. Spike sets through neck and flash hole on primer anvil. Taper of the spike self-limits at the flash hole keeping it from pushing out the dead primer.If you want weight distibuted on more floor area a larger spent shell such as .44spcl/mag/.45acp are very common and easy to trim down w/Weiss snips if needed.