What are you wanting to accomplish with that? Putting something between the spike and the cab? Or on the tip of the cone?
If the former, neoprene washers, If the latter... replace them. No reason to use spikes on hardwood floors, for example. They are best imo for penetrating carpet and coupling with the floor itself. (Why so many spikes come with those spike cups or discs.)
Sorbothane feet or pads, racquetballs, or the industrial style appliance anti-vibration pads (cork or some such material sandwiched by rubber waffles)... I can't speak to the efficacy of them all, but I do use the soundpath feet on my subs and they flippin work. I wasn't having problems with them walking, rather the transference of vibrations to the house structure. Night and day difference! They are claimed to help stop 'walking' subs, too. unless you can find a deal, they're a little more expensive than they should be, but: They Do Work.