You might try www.audiopoints.com or a large hardware store.
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You might contact Sound Anchor at 321-951-4349. Bob Worzella has some long spikes (my stand just came supplied with eight), and I do not think they are expensive. You will need to know what size thread your particular stand requires, that will probably be required info to see if there is stock on hand, and what cost will be.
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.
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.