I really like the spikes come with Martin Logan speakers. You can buy the standard spikes from their web site.
Do they have to be spikes or could you use bolts with same thread? I do this on my hardwood to keep from putting holes in the floor. It seems to work fine if a nut is threaded on the bolt to keep it stable. You can also modify the tip to suit your desires. I admit I'm a cheapskate if you hadn't guessed already.