After I made this post where I made my own Spiked Paddle, reader bft4evr noted that it’s possible to order just the spikes by themselves, and at a much better price!
You can get 100 or 200 packs here. He also posted a few pics of what he’s done with them on his own!
If you are doing the standing board, I’d argue you can do it with a lot less hassle (though admittedly at a higher cost) by just directly getting an acupressure mat. But if you have other ideas (the spiked flip flops, or paddle… or even other ideas you may have), I would agree the loose ones would offer more versatility.
I’m standing in a pair of the modified flip flops right now. It’s painful just to stand, but walking in them is twice as bad. Double the pressure on whichever foot is on the floor. Have your slave do jobs around the house in a pair of these and they won’t forget it.