I ordered them from Bolt Depot. I have a ton of them. I can sell you the bolts. t-nuts, and bit tip you need if you want for a pair of skates for like $10 shipped, I’ll toss in some extras just in case of the bolts and t-nuts.
The t-nut spikes are longer than needed so I trim those down with diagonal cutters. Once I get the hole drilled, I slide the t-nut into the hole as far as it will go inside the boot, and then use the screw to draw the t-bolt down into the sole. It takes a little finesse at times and I have to keep my index finger on the t-nut inside the boot to keep it from spinning. Once it bites in you are good to tighten down. It is careful not to go too tight otherwise you strip out the head of the bolt. In my opinion this system is way superior to rivets.