Where to buy speaker stand spikes ?

Don't waste your time or money. Get some long bolts (either flat head or stove) at your hardware store & screw them in. Same as effect on the sound as spikes

Instead of changing out spikes, could you just raise the speakers and stands onto a platform. I have wooden floors and to protect it, I put the speakers with their original spikes onto two granite slabs. Under the slabs, I use stick-um plastic or rubber feet to isolate the vibration from and to the floor. I bought two 1" thick granite slabs (check with kitchen countertop places, they have scrap granite countertops that they can cut to size for you, I paid $60 for two). In effect, you raise the speakers 1 inch without messing with the spikes. The granite slabs look nice too. You can even match your decors.

Thanks to everyone for the advice. I'm not sure about the option of granite slabs as I have carpeted floor. Just use long bolts is an interesting suggestion, I'll give it a try since it would be the cheapest solution. Thanks again to everyone.

Try Hilti threaded anchors. They have threads on one end and 1" spikes on the other and come in various sizes ie: 1/4", 7/16"... They are used in construction to anchor material to concrete walls and ceilings. They are shot into concrete and the spikes are tough as, well you know. I think Home Base should have them.