Best is to buy rubber feet. I would suspect a hardware store (and likely Walmart, and certainly a woodworking store) would have them. I would avoid spikes on hardwood as it will damage the floor.
For a cheap / quick patch: plot a small mat under each speaker. this will prevent skuffing of the speaker bottom or floor until you can find feet (note: do not use the spikes on the mat, they will go through to the floor).